Nonaqueous electrolytic solution and nonaqueous electrolyte secondary battery using the same

ABSTRACT

Objects of the invention are to provide nonaqueous electrolytic solutions that allow nonaqueous electrolyte secondary batteries to achieve improvements in initial battery characteristics and in battery characteristics after durability testing at the same time, and to provide nonaqueous electrolyte secondary batteries containing the nonaqueous electrolytic solutions. An aspect of the invention resides in a nonaqueous electrolytic solution for a nonaqueous electrolyte secondary battery including a positive electrode and a negative electrode capable of storing and releasing metal ions, the nonaqueous electrolytic solution including an electrolyte and a nonaqueous solvent and further including an aromatic carboxylate ester represented by Formula (1), or an aromatic carboxylate ester (I) represented by Formula (2) and at least one compound (II) selected from the group consisting of fluorine-containing cyclic carbonates, sulfur-containing organic compounds, phosphonate esters, cyano group-containing organic compounds, isocyanate group-containing organic compounds, silicon-containing compounds, aromatic compounds other than those of Formula (2), carboxylate esters represented by Formula (3), cyclic compounds having a plurality of ether bonds, monofluorophosphate salts, difluorophosphate salts, borate salts, oxalate salts and fluorosulfonate salts. Another aspect resides in a nonaqueous electrolyte secondary battery including a negative electrode and a positive electrode capable of storing and releasing lithium ions, and a nonaqueous electrolytic solution including an electrolyte and a nonaqueous solvent, the nonaqueous electrolytic solution being the nonaqueous electrolytic solution described above.

TECHNICAL FIELD

The present invention relates to nonaqueous electrolytic solutions andnonaqueous electrolyte secondary batteries using the same.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries such as lithium secondarybatteries are being actually used in various applications ranging frompower supplies for so-called consumer products such as mobile phones andnotebook computers, to drive power supplies for vehicles such asautomobiles. There have recently been increasing demands on higherperformances for the nonaqueous electrolyte secondary batteries. Inparticular, enhancements are desired in various battery characteristicssuch as high capacity, low-temperature service characteristics,high-temperature storage characteristics, cycle characteristics andovercharge safety.

Electrolytic solutions used in the nonaqueous electrolyte secondarybatteries are usually composed of electrolytes and nonaqueous solventsas the main components. Examples of the main nonaqueous solvents includecyclic carbonates such as ethylene carbonate and propylene carbonate;chain carbonates such as dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate; and cyclic carboxylate esters such as γ-butyrolactoneand γ-valerolactone.

A number of studies have been carried out on nonaqueous solvents,electrolytes and additives in order to improve characteristics ofnonaqueous electrolyte secondary batteries such as load characteristics,cycle characteristics, storage characteristics and overcharge safety.

Patent Literatures 1 to 10 propose that aromatic esters such as methylbenzoate, ethyl benzoate, phenyl propionate, phenyl acetate and benzylacetate are added to electrolytic solutions in order to enhanceproperties of batteries such as energy density, long-term durability,small generation of gas during high-temperature storage andlow-temperature characteristics.

Patent Literature 11 proposes a technique directed to enhancing thesafety of batteries during overcharging by the use of an electrolyticsolution containing a specific carboxylic aromatic ester.

Patent Literature 12 proposes a technique in which a specific carboxylicaromatic ester compound is added to a nonaqueous electrolytic solutionin order to prevent the swelling of batteries during high-temperaturestorage without causing a decrease in battery capacity.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 03463407

Patent Literature 2: Japanese Patent No. 03893627

Patent Literature 3: Japanese Patent Application Kokai Publication No.H10-255836

Patent Literature 4: Japanese Patent Application Kokai Publication No.2000-268831

Patent Literature 5: Japanese Patent Application Kokai Publication No.2005-347222

Patent Literature 6: Japanese Patent No. 02963898

Patent Literature 7: Japanese Patent No. 4051947

Patent Literature 8: Japanese Patent Application Kokai Publication No.2001-297790

Patent Literature 9: Japanese Patent Application Kokai Publication No.2002-033121

Patent Literature 10: Japanese Patent Application Kokai Publication No.2003-338277

Patent Literature 11: Japanese Patent Application Kokai Publication No.2000-058112

Patent Literature 12: Japanese Patent Application Kokai Publication No.2000-173650

Patent Literature 13: WO 2014/003165

DISCLOSURE OF INVENTION Technical Problem

The use of the electrolytic solutions described in Patent Literatures 1to 13 which include compounds having an aromatic group and an estergroup has a problem in that the reactivity of the compounds is so highthat it is difficult for the nonaqueous electrolyte secondary batteriesto achieve enhancement in initial battery characteristics such asinitial capacity, efficiency, rate characteristics and initial gasproduction and at the same time enhancement in battery characteristicsafter durability testing such as capacity after high-temperaturestorage, efficiency, rate characteristics and overcharge safety.

The present invention has been made in order to solve the problemdiscussed above. Therefore, objects of the invention are to providenonaqueous electrolytic solutions that allow nonaqueous electrolytesecondary batteries to achieve improvements in initial batterycharacteristics and in battery characteristics after durability testingat the same time, and to provide nonaqueous electrolyte secondarybatteries containing the nonaqueous electrolytic solutions.

Solution to Problem

The present inventors have conducted various studies to achieve theabove objects. As a result, the present inventors have found that theproblem discussed above can be solved by adding a specific aromaticcarboxylate ester to an electrolytic solution, thereby completing thepresent invention.

The present inventors have further found that the above problem can besolved by the addition of a specific aromatic carboxylate ester and aspecific additive to an electrolytic solution, thereby completing thepresent invention.

The summary of the first aspect of the present invention is describedbelow.

(a) A nonaqueous electrolytic solution for a nonaqueous electrolytesecondary battery including a positive electrode and a negativeelectrode capable of storing and releasing metal ions, the nonaqueouselectrolytic solution including an electrolyte, a nonaqueous solvent andan aromatic carboxylate ester represented by Formula (1):

(in the formula,

A¹ is an optionally substituted aryl group,

n¹ is an integer of 1 or greater,

R² and R³ are independently a hydrogen atom, a halogen atom or anoptionally substituted hydrocarbon group having 1 to 12 carbon atoms andmay be bonded to each other to form a ring, wherein when a plurality ofR²s are present, R²s may be the same as or different from one anotherand when a plurality of R³s are present, R³s may be the same as ordifferent from one another,

a¹ is an integer of 1 or 2,

when a¹ is 1, R¹ is an optionally substituted hydrocarbon group having 1to 12 carbon atoms,

when a¹ is 2, R¹ is an optionally substituted hydrocarbon group having 1to 12 carbon atoms and A's may be the same as or different from eachother,

when n¹ is 1, at least one of R² and R³ is an optionally substitutedhydrocarbon group having 1 to 12 carbon atoms, and

when n¹ is 2, and R²s and R³s are all hydrogen atoms, R¹ is anoptionally substituted aliphatic hydrocarbon group having 1 to 12 carbonatoms).

(b) The nonaqueous electrolytic solution described in (a), wherein a¹ inFormula (1) is 1.

(c) The nonaqueous electrolytic solution described in (a) or (b),wherein A¹ in Formula (1) is a phenyl group.

(d) The nonaqueous electrolytic solution described in any of (a) to (c),wherein the nonaqueous electrolytic solution contains the aromaticcarboxylate ester represented by Formula (1) in 0.001 mass % to 10 mass%.

(e) The nonaqueous electrolytic solution described in any of (a) to (d),wherein the nonaqueous electrolytic solution further includes at leastone compound selected from the group consisting of fluorine-containingcyclic carbonates, sulfur-containing organic compounds, phosphonateesters, cyano group-containing organic compounds, isocyanategroup-containing organic compounds, silicon-containing compounds,aromatic compounds other than those of Formula (1), cyclic carbonateshaving a carbon-carbon unsaturated bond, carboxylate esters other thanthose of Formula (1), cyclic compounds having a plurality of etherbonds, isocyanurate skeleton-containing compounds, monofluorophosphatesalts, difluorophosphate salts, borate salts, oxalate salts andfluorosulfonate salts.

(f) The nonaqueous electrolytic solution described in any of (a) to (e),wherein the nonaqueous electrolytic solution includes 0.001 mass % to 20mass % of at least one compound selected from the group consisting offluorine-containing cyclic carbonates, sulfur-containing organiccompounds, phosphonate esters, cyano group-containing organic compounds,isocyanate group-containing organic compounds, silicon-containingcompounds, aromatic compounds other than those of Formula (1), cycliccarbonates having a carbon-carbon unsaturated bond, carboxylate estersother than those of Formula (1), cyclic compounds having a plurality ofether bonds, isocyanurate skeleton-containing compounds,monofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts.

The summary of the second aspect of the present invention is describedbelow.

(f) A nonaqueous electrolytic solution for a nonaqueous electrolytesecondary battery including a positive electrode and a negativeelectrode capable of storing and releasing metal ions, the nonaqueouselectrolytic solution including an electrolyte, a nonaqueous solvent, anaromatic carboxylate ester (I) and a compound (II),

the aromatic carboxylate ester (I) being represented by Formula (2):

(in the formula,

A² is an optionally substituted aryl group,

n² is an integer of 1 or 2,

a² is an integer of 1 or 2,

when a² is 1, R⁴ is a hydrogen atom, an optionally substituted aliphatichydrocarbon group having 1 to 12 carbon atoms or an optionallysubstituted aryl group, with the proviso that when n² is 2, R⁴ is anoptionally substituted aryl group, and

when a² is 2, R⁴ is a single bond, an optionally substituted aliphatichydrocarbon group having 1 to 12 carbon atoms or an optionallysubstituted arylene group, and A²s may be the same as or different fromeach other, with the proviso that when n² is 2, R⁴ is an optionallysubstituted arylene group),

the compound (II) being at least one selected from the group consistingof fluorine-containing cyclic carbonates, sulfur-containing organiccompounds, phosphonate esters, cyano group-containing organic compounds,isocyanate group-containing organic compounds, silicon-containingcompounds, aromatic compounds other than those of Formula (2),carboxylate esters represented by Formula (3) below, cyclic compoundshaving a plurality of ether bonds, monofluorophosphate salts,difluorophosphate salts, borate salts, oxalate salts and fluorosulfonatesalts,

wherein

R⁵ is a hydrocarbon group having 1 to 4 carbon atoms, and

R⁶ is an ethyl group, an n-propyl group or an n-butyl group.

(g) The nonaqueous electrolytic solution described in (f), wherein a² inFormula (2) is 1.

(h) The nonaqueous electrolytic solution described in (f) or (g),wherein A² in Formula (2) is a phenyl group.

(i) The nonaqueous electrolytic solution described in any of (f) to (h),wherein the nonaqueous electrolytic solution contains the aromaticcarboxylate ester represented by Formula (2) in 0.001 mass % to 10 mass%.

(j) The nonaqueous electrolytic solution described in any of (f) to (i),wherein the nonaqueous electrolytic solution includes 0.001 mass % to 20mass % of the at least one compound selected from the group consistingof fluorine-containing cyclic carbonates, sulfur-containing organiccompounds, phosphonate esters, cyano group-containing organic compounds,isocyanate group-containing organic compounds, silicon-containingcompounds, aromatic compounds other than those of Formula (2),carboxylate esters represented by Formula (3), cyclic compounds having aplurality of ether bonds, monofluorophosphate salts, difluorophosphatesalts, borate salts, oxalate salts and fluorosulfonate salts.

Another aspect of the present invention resides in the following.

(k) A nonaqueous electrolyte secondary battery including a negativeelectrode and a positive electrode capable of storing and releasinglithium ions, and a nonaqueous electrolytic solution including anelectrolyte and a nonaqueous solvent, the nonaqueous electrolyticsolution being the nonaqueous electrolytic solution described in any of(a) to (j).

Advantageous Effects of Invention

The nonaqueous electrolytic solutions according to the present inventionallow nonaqueous electrolyte secondary batteries to achieve bothexcellent initial battery characteristics and excellent batterycharacteristics after durability testing, making it possible to reducethe size of and to enhance the performance and safety of the nonaqueouselectrolyte secondary batteries.

The nonaqueous electrolyte secondary batteries manufactured with theinventive nonaqueous electrolytic solutions, and the inventivenonaqueous electrolyte secondary batteries achieve both excellentinitial battery characteristics and excellent battery characteristicsafter durability testing. The mechanisms and principles for suchsimultaneous enhancement are not fully understood but are probably thefollowing. However, the mechanisms and principles described below do notlimit the scope of the invention.

Aromatic carboxylate esters and carboxylic aromatic esters, typicallythose described in Patent Literatures 1 to 13, usually provide anenhancement in battery characteristics through the formation of filmstructures on positive electrodes. In the aromatic carboxylate esters orthe carboxylic aromatic esters described in Patent Literatures 1 to 10in which an oxycarbonyl group or a carbonyloxy group is bonded directlyon the aromatic ring, however, a vacant orbital of the aromatic ringoverlaps with a vacant orbital of the carbonyl group and consequentlythe compound is reduced very easily on the negative electrode to form alarge amount of a film having low Li⁺ conductivity. As a result, chargedischarge characteristics or charge discharge efficiency at high currentdensity may be significantly decreased. Further, the facilitatedreductive side reaction may cause a significant decrease in dischargecapacity.

In the phenylacetate esters described in Patent Literatures 11 and 12,the carbonyl group and the aromatic ring are so close to each other viaa methylene group that their unoccupied orbitals overlap similarly tothe case of the above compounds. Consequently, the reductive sidereaction takes place on the negative electrode. Thus, these compoundscan cause a decrease in battery characteristics similarly to thecompounds described in Patent Literatures 1 to 10.

Patent Literature 13 is directed to solving the aforementioned problemby using an aromatic carboxylate ester prone to reduction in combinationwith a specific additive. However, the aromatic carboxylate estersdescribed in the literature have so low resistance to oxidation that theside reaction occurs on the positive electrode at a normal servicevoltage possibly to cause a significant decrease in batterycharacteristics.

Meanwhile, at least one compound selected from the group consisting offluorine-containing cyclic carbonates, sulfur-containing organiccompounds, phosphonate esters, cyano group-containing organic compounds,isocyanate group-containing organic compounds, silicon-containingcompounds, aromatic compounds other than those of Formula (2),carboxylate esters represented by Formula (3), cyclic compounds having aplurality of ether bonds, monofluorophosphate salts, difluorophosphatesalts, borate salts, oxalate salts and fluorosulfonate salts (thesecompounds are also written as the compounds (II)) forms a film on thenegative electrode to provide an enhancement in performance. However, adegradation occurs at the same time due to the oxidative side reactionon the positive electrode. Thus, the addition of these compounds aloneprovides only an insufficient enhancement in battery characteristics.

The first aspect of the present invention is based on the finding thatthe above problem can be solved by the incorporation of an aromaticcarboxylate ester represented by Formula (1) into a nonaqueouselectrolytic solution.

As will be described below, the aromatic carboxylate ester representedby Formula (1) has a carbonyl group and an aromatic ring which are soseparated from each other that their vacant orbitals are unlikely tooverlap with each other. This configuration is probably the reason whythe reductive side reaction on the negative electrode is suppressed fromoccurring. Specifically, the aromatic carboxylate ester represented byFormula (1) includes a carbon atom having a hydrocarbon group as asubstituent between the carbonyl group and the aromatic ring. In such astructure, the overlapping of unoccupied orbitals of the carbonyl groupand the aromatic ring is difficult to occur due to the steric hindranceof the hydrocarbon group. As a result, the occurrence of the reductiveside reaction on the negative electrode is reduced to the minimum. Thus,the compound can form a film structure on the positive electrode withoutdecrease in battery characteristics, thereby realizing an enhancement inbattery characteristics. In another embodiment, the aromatic carboxylateester represented by Formula (1) may include a carboxylate skeletonhaving a long alkylene chain between the carbonyl group and the aromaticring. In such a structure, the carbonyl group and the aromatic ring areso separated from each other that the probability of the overlapping oftheir vacant orbitals is reduced. Consequently, the occurrence of thereductive side reaction on the negative electrode can be prevented. Whenthe alkylene group is an ethylene group, the carbonyl group and thearomatic ring are separated from each other but their vacant orbitalshave slight overlapping. Further, in the case where the ester moiety hasan aromatic ring, the compound may have a decreased resistance toreduction because the three groups in the structure, namely, thearomatic ring in the carboxylate skeleton, the carbonyl group and thearomatic ring in the ester moiety interact with one another at the sametime. In this case, the reductive side reaction can take place on thenegative electrode. Thus, the aromatic carboxylate ester represented byFormula (1) has a requirement that when the alkylene group between thecarbonyl group and the aromatic ring in the carboxylate skeleton is anethylene group, the ester moiety is an aliphatic hydrocarbon group.

The second aspect of the present invention is based on the finding thatthe problem discussed hereinabove can be solved by the incorporation ofboth an aromatic carboxylate ester represented by Formula (2) and acompound (II) into a nonaqueous electrolytic solution.

When used alone, the aromatic carboxylate ester represented by Formula(2) will undergo the reductive side reaction on the negative electrodedue to the overlapping of unoccupied orbitals of the aromatic ring andthe carbonyl group in the carboxylate skeleton similarly to as describedhereinabove, and will form a large amount of a film having low Li⁺conductivity. When, in contrast, the aromatic carboxylate esterrepresented by Formula (2) is used together with the compound (II), thecompound (II) forms a film on the negative electrode which prevents thereductive side reaction of the aromatic carboxylate ester represented byFormula (2). Further, part of the aromatic carboxylate ester representedby Formula (2) is incorporated during the formation of a film of thecompound (II) on the negative electrode. Consequently, a stablecomposite film having high Li⁺ conductivity is formed. On the positiveelectrode, the aromatic carboxylate ester represented by Formula (2)forms a film structure which prevents the oxidative side reaction of thecompound (II). As a result, initial battery characteristics and batterycharacteristics after durability testing can be enhanced at the sametime without causing a decrease in battery characteristics.

BEST MODE FOR CARRYING OUT INVENTION

Hereinbelow, embodiments of the invention will be described. However,the scope of the invention is not limited to such embodiments andembraces all modifications without departing from the spirit of theinvention.

1. Nonaqueous Electrolytic Solutions

First Aspect of Invention 1-1. Aromatic Carboxylate Esters Representedby Formula (1)

The first aspect of the invention is characterized in that thenonaqueous electrolytic solution includes an aromatic carboxylate esterrepresented by Formula (1). The aromatic carboxylate ester representedby Formula (1) may be any of optical isomers, that is, may be a singleisomer or a mixture of isomers.

(In the formula,

A¹ is an optionally substituted aryl group,

n¹ is an integer of 1 or greater,

R² and R³ are independently a hydrogen atom, a halogen atom or anoptionally substituted hydrocarbon group having 1 to 12 carbon atoms andmay be bonded to each other to form a ring wherein when a plurality ofR²s are present, R²s may be the same as or different from one anotherand when a plurality of R³s are present, R³s may be the same as ordifferent from one another,

a¹ is an integer of 1 or 2,

when a¹ is 1, R¹ is an optionally substituted hydrocarbon group having 1to 12 carbon atoms,

when a² is 2, R¹ is an optionally substituted hydrocarbon group having 1to 12 carbon atoms and A's may be the same as or different from eachother,

when n¹ is 1, at least one of R² and R³ is a hydrocarbon group having 1to 12 carbon atoms, and

when n¹ is 2 and R²s and R³s are all hydrogen atoms, R¹ is an aliphatichydrocarbon group having 1 to 12 carbon atoms.)

When n¹ in Formula (1) is an integer of 2 or greater and/or when a¹ is2, the compound has a plurality of R²s and a plurality of R³s. Such R²sand R³s each may be the same as or different from one another. When a¹is 2, the plurality of A¹s may be the same as or different from eachother.

In Formula (1), R² and R³ may be bonded to each other to form a ring. Inthis case, it is preferable that R² and R³ on the same carbon atom bebonded to each other to form a ring. In Formula (1), no rings are formedby the bonding of R¹ and R², the bonding of R¹ and R³, the bonding of A¹and R¹, the bonding of A¹ and R², the bonding of A¹ and R³, the bondingof R¹, R² and A¹, or the bonding of R¹, R³ and A¹.

In Formula (1), R¹ is an optionally substituted hydrocarbon group having1 to 12 carbon atoms. When a¹ is 1, R¹ is a monovalent group. When a¹ is2, R¹ is a divalent group. In the optionally substituted hydrocarbongroup having 1 to 12 carbon atoms, the number of carbon atoms rangingfrom 1 to 12 refers to the number of carbon atoms in the hydrocarbongroup. When R¹ has a substituent, the above number of carbon atoms doesnot include the number of carbon atoms in the substituent. The number ofcarbon atoms in the hydrocarbon group R¹ is preferably not more than 10,more preferably not more than 9, and still more preferably not more than5.

Here, the term hydrocarbon groups indicates groups composed of carbonand hydrogen atoms, and specifically refers to aliphatic hydrocarbongroups and aromatic hydrocarbon groups. The aliphatic hydrocarbon groupsare acyclic or cyclic hydrocarbon groups composed of carbon and hydrogenatoms without any aromatic structures. The aromatic hydrocarbon groupsare hydrocarbon groups composed of carbon and hydrogen atoms with anaromatic structure.

Here, the term substituents refers to groups composed of one or moreatoms selected from the group consisting of carbon atoms, hydrogenatoms, nitrogen atoms, oxygen atoms, sulfur atoms, phosphorus atoms andhalogen atoms (except those groups composed solely of carbon andhydrogen atoms).

Examples of the substituents include halogen atoms (preferably fluorineatoms); alkoxy groups; halogenated (preferably fluorinated) alkylgroups, alkenyl groups, alkynyl groups, aryl groups or alkoxy groups;cyano groups; isocyanate groups; alkoxycarbonyloxy groups; acyl groups;carboxyl groups; alkoxycarbonyl groups; acyloxy groups; alkylsulfonylgroups; alkoxysulfonyl groups; dialkoxyphosphanetriyl groups;dialkoxyphosphoryl groups; and dialkoxyphosphoryloxy groups. Halogenatoms and halogenated alkyl groups are preferable, and halogen atoms andhalogenated alkyl groups are more preferable. Examples of the alkylgroups and the alkoxy groups as the substituents (including such groupsconstituting parts of the substituents) include those groups having 1 to6 carbon atoms. Examples of the alkenyl groups and the alkynyl groupsinclude those groups having 2 to 6 carbon atoms. Examples of the arylgroups include those groups having 6 to 12 carbon atoms.

Examples of the hydrocarbon groups will be described below. While thefollowing examples illustrate monovalent hydrocarbon groups(corresponding to when a¹ is 1), the corresponding divalent groups maybe adopted when a¹ is 2. For example, the divalent groups correspondingto alkyl groups, alkenyl groups, alkynyl groups and aryl groups arealkylene groups, alkenylene groups, alkynylene groups and arylenegroups, respectively.

Examples of the hydrocarbon groups include aliphatic hydrocarbon groupssuch as alkyl groups, alkenyl groups and alkynyl groups, and aromatichydrocarbon groups such as aryl groups and aralkyl groups.

Of these, preferred groups are, for example, alkyl groups having 1 to 5carbon atoms such as methyl group, ethyl group, n-propyl group, i-propylgroup, n-butyl group, sec-butyl group, i-butyl group, tert-butyl group,n-pentyl group, isopentyl group, sec-pentyl group, neopentyl group,1-methylbutyl group, 2-methylbutyl group, 1,1-dimethylpropyl group and1,2-dimethylpropyl group; alkenyl groups having 2 to 5 carbon atoms suchas vinyl group, 1-propenyl group, 2-propenyl group, isopropenyl group,1-butenyl group, 2-butenyl group, 3-butenyl group, 1-pentenyl group,2-pentenyl group, 3-pentenyl group and 4-pentenyl group; alkynyl groupshaving 2 to 5 carbon atoms such as ethynyl group, 1-propynyl group,2-propynyl group, 1-butynyl group, 2-butynyl group, 3-butynyl group,1-pentynyl group, 2-pentynyl group, 3-pentynyl group and 4-pentynylgroup; aryl groups such as phenyl group, tolyl group, xylyl group,ethylphenyl group, n-propylphenyl group, i-propylphenyl group,n-butylphenyl group, sec-butylphenyl group, i-butylphenyl group andtert-butylphenyl group; and aralkyl groups having 7 to 12 carbon atomssuch as benzyl group, α-methylbenzyl group, 1-methyl-1-phenylethylgroup, phenethyl group, 2-phenylpropyl group, 2-methyl-2-phenylpropylgroup, 3-phenylpropyl group, 3-phenylbutyl group, 3-methyl-3-phenylbutylgroup, 4-phenylbutyl group, 5-phenylpentyl group and 6-phenylhexylgroup. More preferred groups are alkyl groups having 1 to 5 carbon atomssuch as methyl group, ethyl group, n-propyl group, i-propyl group,n-butyl group, sec-butyl group, i-butyl group, tert-butyl group,n-pentyl group, isopentyl group, sec-pentyl group, neopentyl group,1-methylbutyl group, 2-methylbutyl group, 1,1-dimethylpropyl group and1,2-dimethylpropyl group; and aralkyl groups having 7 to 12 carbon atomssuch as benzyl group, α-methylbenzyl group, 1-methyl-1-phenylethylgroup, phenethyl group, 2-phenylpropyl group, 2-methyl-2-phenylpropylgroup, 3-phenylpropyl group, 3-phenylbutyl group, 3-methyl-3-phenylbutylgroup, 4-phenylbutyl group, 5-phenylpentyl group and 6-phenylhexylgroup. Methyl group, ethyl group, n-propyl group, n-butyl group, benzylgroup, phenethyl group, 3-phenylpropyl group and 4-phenylbutyl group arestill more preferable, and methyl group, ethyl group, n-propyl group andn-butyl group are particularly preferable.

Substituted hydrocarbon groups are also suitably used. Examples of thesubstituents include halogen atoms (preferably fluorine atoms) andunsubstituted or halogenated (preferably fluorinated) alkoxy groups.Preferred examples of the substituted hydrocarbon groups includetrifluoromethylphenyi group, methoxyphenyl group, ethoxyphenyl group,trifluoromethoxyphenyl group, monofluorophenyl group, difluorophenylgroup, trifluorophenyl group and tetrafluorophenyl group.

While the above examples illustrate monovalent hydrocarbon groups andmonovalent substituted hydrocarbon groups, the corresponding divalentgroups may be adopted when a¹ is 2. In particular, preferred examplesinclude alkylene groups having 1 to 5 carbon atoms such as methylenegroup, ethylene group, trimethylene group, tetramethylene group andpentamethylene group; alkenylene groups having 2 to 5 carbon atoms suchas vinylene group, 1-propenylene group, 2-propenylene group,1-butenylene group, 2-butenylene group, 1-pentenylene group and2-pentenylene group; and alkynylene groups having 2 to 5 carbon atomssuch as ethynylene group, propynylene group, 1-butynylene group,2-butynylene group, 1-pentylene group and 2-pentylene group.

In Formula (1), R² and R³ are independently a hydrogen atom, a halogenatom (preferably a fluorine atom) or an optionally substitutedhydrocarbon group having 1 to 12 carbon atoms and may be bonded to eachother to form a ring. Examples of the substituents include those groupsmentioned for R¹. In particular, they are preferably each a hydrogenatom or an optionally substituted hydrocarbon group having 1 to 12carbon atoms, more preferably each an optionally substituted hydrocarbongroup having 1 to 12 carbon atoms, and still more preferably each anunsubstituted hydrocarbon group having 1 to 12 carbon atoms.

When R² and R³ are optionally substituted hydrocarbon groups, the numberof carbon atoms in the hydrocarbon group is preferably not more than 8,more preferably not more than 4, and still more preferably not more than2.

Examples of the hydrocarbon groups include aliphatic hydrocarbon groupssuch as alkyl groups, alkenyl groups and alkynyl groups, and aromatichydrocarbon groups such as aryl groups and aralkyl groups.

Of these, preferred groups are alkyl groups having 1 to 4 carbon atomssuch as methyl group, ethyl group, n-propyl group, i-propyl group,n-butyl group, sec-butyl group, i-butyl group and tert-butyl group;alkenyl groups having 2 to 4 carbon atoms such as vinyl group,1-propenyl group, 2-propenyl group, isopropenyl group, 1-butenyl group,2-butenyl group and 3-butenyl group; alkynyl groups having 2 to 4 carbonatoms such as ethynyl group, 1-propynyl group, 2-propynyl group,1-butynyl group, 2-butynyl group and 3-butynyl group; aryl groups suchas phenyl group, tolyl group, xylyl group, ethylphenyl group,n-propylphenyl group, i-propylphenyl group, n-butylphenyl group,sec-butylphenyl group, i-butylphenyl group and tert-butylphenyl group;and aralkyl groups having 7 to 12 carbon atoms such as benzyl group,α-methylbenzyl group, 1-methyl-1-phenylethyl group, phenethyl group,2-phenylpropyl group, 2-methyl-2-phenylpropyl group, 3-phenylpropylgroup, 3-phenylbutyl group, 3-methyl-3-phenylbutyl group, 4-phenylbutylgroup, 5-phenylpentyl group and 6-phenylhexyl group. Alkyl groups having1 to 4 carbon atoms such as methyl group, ethyl group, n-propyl group,i-propyl group, n-butyl group, sec-butyl group, i-butyl group andtert-butyl group are more preferable, and methyl group and ethyl groupare still more preferable.

Substituted hydrocarbon groups are also suitably used. Examples of thesubstituents include halogen atoms (preferably fluorine atoms) andunsubstituted or halogenated (preferably fluorinated) alkoxy groups.Specific examples include trifluoromethylphenyl group, methoxyphenylgroup, ethoxyphenyl group and trifluoromethoxyphenyl group.

When R² and R³ are bonded to each other to form a ring, the ringstructure is not particularly limited but is preferably such that thering skeleton is composed of carbon, nitrogen, oxygen and sulfur atoms,and more preferably composed of carbon atoms. The number of ring memberatoms in the ring structure may be 3 to 12, and preferably 4 to 8. Whenthe ring skeleton is composed of carbon atoms, the total number ofcarbon atoms constituting the ring may be 3 or more, and preferably 4 ormore, and may be 12 or less, preferably 8 or less, more preferably 6 orless, and still more preferably 5 or less.

Specific examples of the ring structures include cycloalkane structures,oxacycloalkane structures, azacycloalkane structures and thiacycloalkanestructures. Of these, cycloalkane structures having 3 to 12 carbon atomsare preferable, with examples including cyclopropane structure,cyclobutane structure, cyclopentane structure, cyclohexane structure,cycloheptane structure, cyclooctane structure, cyclononane structure,cyclodecane structure, cycloundecane structure and cyclododecanestructure. Cyclopropane structure, cyclobutane structure, cyclopentanestructure and cyclohexane structure are more preferable, andcyclopentane structure and cyclohexane structure are still morepreferable.

In Formula (1), A¹ is an optionally substituted aryl group. Examples ofthe substituents include those groups mentioned for R¹. The aryl groupsare not particularly limited, but the number of carbon atoms may be 6 ormore, preferably 7 or more, and more preferably 8 or more, and may be 12or less, preferably 11 or less, and more preferably 10 or less.

Examples of the aryl groups include phenyl group, tolyl group,ethylphenyl group, n-propylphenyl group, i-propylphenyl group,n-butylphenyl group, sec-butylphenyl group, i-butylphenyl group,tert-butylphenyl group and xylyl group. Those aryl groups having halogenatoms (preferably fluorine atoms) or unsubstituted or halogenated(preferably fluorinated) alkoxy groups as the substituents are alsopreferable, with examples including trifluoromethylphenyl group,methoxyphenyl group, ethoxyphenyl group, trifluoromethoxyphenyl group,monofluorophenyl group, difluorophenyl group, trifluorophenyl group,tetrafluorophenyl group and pentafluorophenyl group. Of these, phenylgroup, tolyl group, tert-butylphenyl group, methoxyphenyl group andmonofluorophenyl group are preferable. Phenyl group and tolyl group aremore preferable, and phenyl group is still more preferable.

In Formula (1), n¹ is an integer of 1 or greater and may be an integerof 5 or less, preferably 4 or less, more preferably 3 or less, stillmore preferably 2 or less, and particularly preferably 1.

In Formula (1), a¹ is an integer of 1 or 2, and is preferably 1.

When n¹ is 1, at least one of R² and R³ is a hydrocarbon group having 1to 12 carbon atoms. When n¹ is 2 and R²s and R³s are all hydrogen atoms,R¹ is an aliphatic hydrocarbon group having 1 to 12 carbon atoms.

Examples of the aromatic carboxylate esters represented by Formula (1)include the following compounds.

a¹=1

Specific examples of the compounds in which a¹=1 are illustrated below.In the examples, R¹ represents a hydrocarbon group selected from methylgroup, ethyl group, n-propyl group, i-propyl group, n-butyl group,sec-butyl group, i-butyl group, tert-butyl group, n-pentyl group,isopentyl group, sec-pentyl group, neopentyl group, 1-methylbutyl group,2-methylbutyl group, 1,1-dimethylpropyl group, 1,2-dimethylpropyl group,benzyl group, α-methylbenzyl group, 1-methyl-1-phenylethyl group,phenethyl group, 2-phenylpropyl group, 2-methyl-2-phenylpropyl group,3-phenylpropyl group, 3-phenylbutyl group, 3-methyl-3-phenylbutyl group,4-phenylbutyl group, 5-phenylpentyl group and 6-phenylhexyl group, andR¹ is a hydrocarbon group selected from methyl group, ethyl group,n-propyl group, i-propyl group, n-butyl group, sec-butyl group, i-butylgroup, tert-butyl group, n-pentyl group, isopentyl group, sec-pentylgroup, neopentyl group, 1-methylbutyl group, 2-methylbutyl group,1,1-dimethylpropyl group and 1,2-dimethylpropyl group.

-   -   n¹=1, R²=halogen atom or optionally halogenated hydrocarbon        group, R³=hydrogen atom, A¹=phenyl group

-   -   n¹=1, R²=halogen atom or hydrocarbon group, R³=hydrogen atom,        A¹=tolyl group

-   -   n¹=1, R²=halogen atom or hydrocarbon group, R³=halogen atom or        hydrocarbon group, A¹=phenyl group

-   -   n¹=1, R² and R³ bonded to each other to form a ring, A¹=phenyl        group

-   -   n¹=1, R²=halogen atom or hydrocarbon group, R³=halogen atom or        hydrocarbon group, A¹=tolyl group

-   -   n¹=1, R² and R³ bonded to each other to form a ring, A¹=tolyl        group

-   -   n¹=2

-   -   n¹=3

-   -   n¹≥4

a¹=2

Specific examples of the compounds in which a¹=2 are illustrated below.In the examples, R represents a hydrocarbon group selected frommethylene group, ethylene group, trimethylene group, tetramethylenegroup, pentamethylene group, vinylene group, 1-propenylene group,2-propenylene group, 1-butenylene group, 2-butenylene group,1-pentenylene group, 2-pentenylene group, ethynylene group, propynylenegroup, 1-butynylene group, 2-butynylene group, 1-pentylene group and2-pentylene group.

In particular, the following compounds are preferable in view of thefact that they are less prone to the reductive side reaction on thenegative electrode.

Of the above compounds, the following compounds are more preferable inview of the fact that they are less prone to the oxidative side reactionon the positive electrode.

The aromatic carboxylate esters represented by Formula (1) may be usedsingly, or two or more may be used in combination. In the whole amountof the nonaqueous electrolytic solution (100 mass %), the amount of thearomatic carboxylate ester represented by Formula (1) (the total amountwhen two or more kinds of the esters are used) may be 0.001 mass % orabove, preferably 0.01 mass % or above, more preferably 0.05 mass % orabove, and still more preferably 0.1 mass % or above, and may be 10 mass% or below, preferably 8 mass % or below, more preferably 5 mass % orbelow, still more preferably 3 mass % or below, and particularlypreferably 2.5 mass % or below. This amount ensures that theadvantageous effects of the invention will be obtained prominently andthe increase in battery resistance can be prevented.

The compounds described above may be added to the electrolytic solutionof the invention by any methods without limitation. For example, thecompounds may be added directly to the electrolytic solution, or amethod may be adopted which generates the compounds in the battery or inthe electrolytic solution. For example, the compounds may be generatedby adding precursor compounds followed by reaction such as oxidation orhydrolysis of the battery components such as the electrolytic solution.Alternatively, the compounds may be generated in the battery by theapplication of electric loads such as charging and discharging.

In the first aspect of the present invention, the aromatic carboxylateester represented by Formula (1) may be used in combination with atleast one compound selected from the group consisting of aromaticcarboxylate esters represented by Formula (2) described later, cycliccarbonates having a carbon-carbon unsaturated bond and compounds (II)described later. Of the compounds (II), the aromatic compounds otherthan those of Formula (2) do not include the aromatic compoundsrepresented by Formula (1). And carboxylate esters represented byFormula (3) are other than the carboxylate esters of Formula (1).Specific examples and preferred examples of the aromatic carboxylateesters represented by Formula (2) and the compounds (II) as well aspreferred amounts in which these compounds are used will be described inthe second aspect of the present invention. Examples of the cycliccarbonates having a carbon-carbon unsaturated bond include vinylenecarbonate, and the description in “1-6. Auxiliaries” also applies to thecyclic carbonates used here.

Second Aspect of Invention 1-2. Aromatic Carboxylate Esters Representedby Formula (2)

The second aspect of the invention is characterized in that thenonaqueous electrolytic solution includes an aromatic carboxylate esterrepresented by Formula (2). The aromatic carboxylate ester representedby Formula (2) may be any of optical isomers, that is, may be a singleisomer or a mixture of isomers.

(In the formula,

A² is an optionally substituted aryl group,

n² is an integer of 1 or 2,

a² is an integer of 1 or 2,

when a² is 1, R⁴ is a hydrogen atom, an optionally substituted aliphatichydrocarbon group having 1 to 12 carbon atoms or an optionallysubstituted aryl group, with the proviso that when n² is 2, R⁴ is anoptionally substituted aryl group, and

when a² is 2, R⁴ is a single bond, an optionally substituted aliphatichydrocarbon group having 1 to 12 carbon atoms or an optionallysubstituted arylene group, and A²s may be the same as or different fromeach other, with the proviso that when n² is 2, R⁴ is an optionallysubstituted arylene group.)

In Formula (2), R⁴ and A² are not bonded to each other to form a ring.

When a² in Formula (2) is 1, R⁴ is a hydrogen atom, an optionallysubstituted (monovalent) aliphatic hydrocarbon group having 1 to 12carbon atoms or an optionally substituted aryl group. Examples of thesubstituents include those groups mentioned for R¹.

The number of carbon atoms in the (monovalent) aliphatic hydrocarbongroup is preferably 10 or less, more preferably 9 or less, and stillmore preferably 5 or less.

Examples of the (monovalent) aliphatic hydrocarbon groups include alkylgroups, alkenyl groups and alkynyl groups.

Of these, preferred groups are, for example, alkyl groups having 1 to 5carbon atoms such as methyl group, ethyl group, n-propyl group, i-propylgroup, n-butyl group, sec-butyl group, i-butyl group, tert-butyl group,n-pentyl group, isopentyl group, sec-pentyl group, neopentyl group,1-methylbutyl group, 2-methylbutyl group, 1,1-dimethylpropyl group and1,2-dimethylpropyl group; alkenyl groups having 2 to 5 carbon atoms suchas vinyl group, 1-propenyl group, 2-propenyl group, isopropenyl group,1-butenyl group, 2-butenyl group, 3-butenyl group, 1-pentenyl group,2-pentenyl group, 3-pentenyl group and 4-pentenyl group; and alkynylgroups having 2 to 5 carbon atoms such as ethynyl group, 1-propynylgroup, 2-propynyl group, 1-butynyl group, 2-butynyl group, 3-butynylgroup, 1-pentynyl group, 2-pentynyl group, 3-pentynyl group and4-pentynyl group. More preferred groups are alkyl groups having 1 to 5carbon atoms such as methyl group, ethyl group, n-propyl group, i-propylgroup, n-butyl group, sec-butyl group, i-butyl group, tert-butyl group,n-pentyl group, isopentyl group, sec-pentyl group, neopentyl group,1-methylbutyl group, 2-methylbutyl group, 1,1-dimethylpropyl group and1,2-dimethylpropyl group. Methyl group, ethyl group, n-propyl group andn-butyl group are still more preferable, and methyl group and ethylgroup are particularly preferable.

The aliphatic hydrocarbon group may have a substituent but is preferablyunsubstituted.

The aryl groups are not particularly limited, but the number of carbonatoms may be 6 or more, preferably 7 or more, and more preferably 8 ormore, and may be 12 or less, preferably 11 or less, and more preferably10 or less.

Examples of the aryl groups include phenyl group, tolyl group,ethylphenyl group, n-propylphenyl group, i-propylphenyl group,n-butylphenyl group, sec-butylphenyl group, i-butylphenyl group,tert-butylphenyl group and xylyl group. Those aryl groups having halogenatoms (preferably fluorine atoms) or unsubstituted or halogenated(preferably fluorinated) alkoxy groups as the substituents are alsopreferable, with examples including trifluoromethylphenyl group, xylylgroup, methoxyphenyl group, ethoxyphenyl group, trifluoromethoxyphenylgroup, monofluorophenyl group, difluorophenyl group, trifluorophenylgroup, tetrafluorophenyl group and pentafluorophenyl group. Of these,phenyl group, tolyl group, tert-butylphenyl group, methoxyphenyl groupand monofluorophenyl group are preferable. Phenyl group and tolyl groupare more preferable, and phenyl group is still more preferable.

When a² is 1, R⁴ is preferably a hydrogen atom or an optionallysubstituted (monovalent) aliphatic hydrocarbon group having 1 to 12carbon atoms, more preferably a hydrogen atom or an unsubstituted(monovalent) aliphatic hydrocarbon group having 1 to 12 carbon atoms,and still more preferably a hydrogen atom. In this case, when n² is 2,R⁴ is an optionally substituted aryl group, and is preferably anunsubstituted aryl group.

When a² in Formula (2) is 2, R⁴ is a single bond, an optionallysubstituted (divalent) aliphatic hydrocarbon group having 1 to 12 carbonatoms or an optionally substituted arylene group.

Examples of the (divalent) aliphatic hydrocarbon groups include thosedivalent groups corresponding to the (monovalent) aliphatic hydrocarbongroups described above. In particular, for example, preferred groups arealkylene groups having 1 to 5 carbon atoms such as methylene group,ethylene group, trimethylene group, tetramethylene group andpentamethylene group; alkenylene groups having 2 to 5 carbon atoms suchas vinylene group, 1-propenylene group, 2-propenylene group,1-butenylene group, 2-butenylene group, 1-pentenylene group and2-pentenylene group; and alkynylene groups having 2 to 5 carbon atomssuch as ethynylene group, propynylene group, 1-butynylene group,2-butynylene group, 1-pentylene group and 2-pentylene group.

Examples of the arylene groups include those divalent groupscorresponding to the aryl groups described above. In particular, forexample, phenylene group is preferable.

When a² is 2, R⁴ is preferably a single bond or an optionallysubstituted (divalent) aliphatic hydrocarbon group having 1 to 12 carbonatoms, and more preferably a single bond or an unsubstituted (divalent)aliphatic hydrocarbon group having 1 to 12 carbon atoms. In this case,when n² is 2, R⁴ is an optionally substituted arylene group, and ispreferably an unsubstituted arylene group.

In Formula (2), A² is an optionally substituted aryl group. Examples ofthe substituents include those groups mentioned for R1. The aryl groupsare not particularly limited, but the number of carbon atoms may be 6 ormore, preferably 7 or more, and more preferably 8 or more, and may be 12or less, preferably 11 or less, and more preferably 10 or less. Examplesof the aryl groups include phenyl group, tolyl group, ethylphenyl group,n-propylphenyl group, i-propylphenyl group, n-butylphenyl group,sec-butylphenyl group, i-butylphenyl group, tert-butylphenyl group andxylyl group. Those aryl groups having halogen atoms (preferably fluorineatoms) or unsubstituted or halogenated (preferably fluorinated) alkoxygroups as the substituents are also preferable, with examples includingtrifluoromethylphenyl group, xylyl group, methoxyphenyl group,ethoxyphenyl group, trifluoromethoxyphenyl group, monofluorophenylgroup, difluorophenyl group, trifluorophenyl group, tetrafluorophenylgroup and pentafluorophenyl group. Of these, phenyl group, tolyl group,tert-butylphenyl group, methoxyphenyl group and monofluorophenyl groupare preferable. Phenyl group and tolyl group are more preferable, andphenyl group is still more preferable.

In Formula (2), n² is preferably 1. In Formula (2), a² is preferably 1.

Examples of the aromatic carboxylate esters represented by Formula (2)include the following compounds.

a²=1

Specific examples of the compounds in which a²=1 are illustrated below.In the examples, R represents a hydrogen atom or a group selected frommethyl group, ethyl group, n-propyl group, i-propyl group, n-butylgroup, sec-butyl group, i-butyl group, tert-butyl group, n-pentyl group,isopentyl group, sec-pentyl group, neopentyl group, 1-methylbutyl group,2-methylbutyl group, 1,1-dimethylpropyl group, 1,2-dimethylpropyl group,phenyl group, tolyl group, tert-butylphenyl group, methoxyphenyl groupand monofluorophenyl group, and R′ is an aryl group selected from phenylgroup, tolyl group, tert-butylphenyl group, methoxyphenyl group andmonofluorophenyl group.

a²=2

Specific examples of the compounds in which a²=2 are illustrated below.In the examples, R represents a hydrocarbon group selected frommethylene group, ethylene group, trimethylene group, tetramethylenegroup, pentamethylene group, vinylene group and ethynylene group.

In particular, the following compounds are preferable in view of thefact that the compound is less prone to the reductive side reaction onthe negative electrode.

Of the above compounds, the following compound is more preferable inview of the fact that the compound is less prone to the oxidative sidereaction on the positive electrode.

The aromatic carboxylate esters represented by Formula (2) may be usedsingly, or two or more may be used in combination. In the whole amountof the nonaqueous electrolytic solution (100 mass %), the amount of thearomatic carboxylate ester represented by Formula (2) (the total amountwhen two or more kinds of the esters are used) may be 0.001 mass % orabove, preferably 0.01 mass % or above, more preferably 0.05 mass % orabove, and still more preferably 0.1 mass % or above, and may be 10 mass% or below, preferably 8 mass % or below, more preferably 5 mass % orbelow, still more preferably 3 mass % or below, and particularlypreferably 2.5 mass % or below. This amount ensures that theadvantageous effects of the invention will be obtained prominently andthe increase in battery resistance can be prevented.

The compounds described above may be added to the electrolytic solutionof the invention by any methods without limitation. For example, thecompounds may be added directly to the electrolytic solution, or amethod may be adopted which generates the compounds in the battery or inthe electrolytic solution. For example, the compounds may be generatedby adding precursor compounds followed by reaction such as oxidation orhydrolysis of the battery components such as the electrolytic solution.Alternatively, the compounds may be generated in the battery by theapplication of electric loads such as charging and discharging.

1-3. Compounds (II)

The second aspect of the present invention is characterized in that thenonaqueous electrolytic solution includes the aromatic carboxylate esterrepresented by Formula (2) together with at least one compound (compound(II)) selected from the group consisting of fluorine-containing cycliccarbonates, sulfur-containing organic compounds, phosphonate esters,cyano group-containing organic compounds, isocyanate group-containingorganic compounds, silicon-containing compounds, aromatic compoundsother than those of Formula (2), carboxylate esters represented byFormula (3), cyclic compounds having a plurality of ether bonds,monofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts. The combined use makes itpossible to efficiently prevent the aromatic carboxylate esterrepresented by Formula (2) from possible side reactions.

In particular, a quality composite film is advantageously formed on thenegative electrode and initial battery characteristics and batterycharacteristics after durability testing are advantageously enhanced ina balanced manner by using at least one compound selected from the groupconsisting of fluorine-containing cyclic carbonates, sulfur-containingorganic compounds, phosphonate esters, cyano group-containing organiccompounds, isocyanate group-containing organic compounds,silicon-containing compounds, aromatic compounds other than those ofFormula (2), carboxylate esters represented by Formula (3),monofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts. It is more preferable that thecompound be at least one selected from the group consisting offluorine-containing cyclic carbonates, sulfur-containing organiccompounds, phosphonate esters, cyano group-containing organic compounds,isocyanate group-containing organic compounds, silicon-containingcompounds, aromatic compounds other than those of Formula (2),carboxylate esters represented by Formula (3), monofluorophosphatesalts, difluorophosphate salts, oxalate salts and fluorosulfonate salts.It is still more preferable that the compound be at least one selectedfrom the group consisting of fluorine-containing cyclic carbonates,sulfur-containing organic compounds, phosphonate esters, cyanogroup-containing organic compounds, isocyanate group-containing organiccompounds, silicon-containing compounds, aromatic compounds other thanthose of Formula (2), monofluorophosphate salts, difluorophosphatesalts, oxalate salts and fluorosulfonate salts. It is particularlypreferable that the compound be at least one selected from the groupconsisting of sulfur-containing organic compounds, phosphonate esters,cyano group-containing organic compounds, isocyanate group-containingorganic compounds, silicon-containing compounds, monofluorophosphatesalts, difluorophosphate salts, oxalate salts and fluorosulfonate salts.One of the reasons for this preference is because these compounds formrelatively low-molecular weight films on the negative electrodes and thenegative electrode films formed are so dense that the films canefficiently prevent the aromatic carboxylate ester of Formula (2) frombeing degraded by side reactions. In this manner, the use of the abovecompounds effectively suppresses the occurrence of side reactions andalso prevents the increase in resistance. The suppression of sidereactions during initial stages or during long exposure to hightemperatures makes it possible to suppress the occurrence of volumechange and to ensure safety after long exposure to high temperatures andalso makes it possible to enhance rate characteristics.

The compounds described above may be added to the electrolytic solutionof the invention by any methods without limitation. For example, thecompounds may be added directly to the electrolytic solution, or amethod may be adopted which generates the compounds in the battery or inthe electrolytic solution. For example, the compounds may be generatedby adding precursor compounds followed by reaction such as oxidation orhydrolysis of the battery components such as the electrolytic solution.Alternatively, the compounds may be generated in the battery by theapplication of electric loads such as charging and discharging.

Hereinbelow, the compounds (II) will be described. For themonofluorophosphate salts, the difluorophosphate salts, the boratesalts, the oxalate salts and the fluorosulfonate salts, reference may bemade to the description in “1-4. Electrolytes”.

1-3-1. Fluorine-Containing Cyclic Carbonates

Examples of the fluorine-containing cyclic carbonates include fluoridesof cyclic carbonates having an alkylene group with 2 to 6 carbon atoms,and derivatives thereof, such as ethylene carbonate fluoride(hereinafter, also written as “fluorinated ethylene carbonate”) andderivatives thereof. Examples of the derivatives of ethylene carbonatefluoride include ethylene carbonate fluorides substituted with an alkylgroup (for example, an alkyl group having 1 to 4 carbon atoms). Inparticular, fluorinated ethylene carbonates and derivatives thereofhaving 1 to 8 fluorine atoms are preferred.

In the electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and thefluorine-containing cyclic carbonate reduces the initial gas productionin a battery containing the electrolytic solution, and also increasesthe overcharge gas production in the battery to make it possible tofurther enhance the battery safety.

Examples of the fluorinated ethylene carbonates and the derivativesthereof having 1 to 8 fluorine atoms include monofluoroethylenecarbonate, 4,4-difluoroethylene carbonate, 4,5-difluoroethylenecarbonate, 4-fluoro-4-methylethylene carbonate,4,5-difluoro-4-methylethylene carbonate, 4-fluoro-5-methylethylenecarbonate, 4,4-difluoro-5-methylethylene carbonate,4-(fluoromethyl)-ethylene carbonate, 4-(difluoromethyl)-ethylenecarbonate, 4-(trifluoromethyl)-ethylene carbonate,4-(fluoromethyl)-4-fluoroethylene carbonate,4-(fluoromethyl)-5-fluoroethylene carbonate,4-fluoro-4,5-dimethylethylene carbonate,4,5-difluoro-4,5-dimethylethylene carbonate and4,4-difluoro-5,5-dimethylethylene carbonate.

In particular, monofluoroethylene carbonate, 4,4-difluoroethylenecarbonate and 4,5-difluoroethylene carbonate are preferable because theygive high ion conductivity to the electrolytic solution and easily forma stable interface protective film.

The fluorinated cyclic carbonate may be used singly, or two or more maybe used in combination in an appropriate ratio. In 100 mass % of theelectrolytic solution, the amount of the fluorinated cyclic carbonate(the total amount when two or more kinds of the carbonates are used) ispreferably 0.001 mass % or above, more preferably 0.01 mass % or above,still more preferably 0.1 mass % or above, and even more preferably 0.4mass % or above, and is preferably 10 mass % or less, more preferably 7mass % or less, still more preferably 5 mass % or less, particularlypreferably 3 mass % or less, and most preferably 1.5 mass % or less. Inthe case where the fluorinated cyclic carbonate is used as a nonaqueoussolvent, the amount thereof in 100 vol % of the nonaqueous solvent(s) ispreferably 1 vol % or above, more preferably 5 vol % or above, and stillmore preferably 10 vol % or above, and is preferably 50 vol % or less,more preferably 35 vol % or less, and still more preferably 25 vol % orless.

The fluorine-containing cyclic carbonates may be cyclic carbonateshaving an unsaturated bond and a fluorine atom (hereinafter, alsowritten as “fluorinated unsaturated cyclic carbonates”). The fluorinatedunsaturated cyclic carbonates may have one or more fluorine atomswithout limitation. The number of fluorine atoms may be 6 or less,preferably 4 or less, and more preferably 1 or 2.

Examples of the fluorinated unsaturated cyclic carbonates includefluorinated vinylene carbonate derivatives, and fluorinated ethylenecarbonate derivatives substituted with a substituent having an aromaticring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate,4-fluoro-5-phenylvinylene carbonate, 4-allyl-5-fluorovinylene carbonateand 4-fluoro-5-vinylvinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith a substituent having an aromatic ring or a carbon-carbon doublebond include 4-fluoro-4-vinylethylene carbonate,4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinylethylene carbonate,4-fluoro-5-allylethylene carbonate, 4,4-difluoro-4-vinylethylenecarbonate, 4,4-difluoro-4-allylethylene carbonate,4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allylethylenecarbonate, 4-fluoro-4,5-divinylethylene carbonate,4-fluoro-4,5-diallylethylene carbonate, 4,5-difluoro-4,5-divinylethylenecarbonate, 4,5-difluoro-4,5-diallylethylene carbonate,4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylenecarbonate, 4,4-difluoro-5-phenylethylene carbonate and4,5-difluoro-4-phenylethylene carbonate.

In particular, 4-fluorovinylene carbonate, 4-fluoro-5-methylvinylenecarbonate, 4-fluoro-5-vinylvinylene carbonate, 4-allyl-5-fluorovinylenecarbonate, 4-fluoro-4-vinylethylene carbonate, 4-fluoro-4-allylethylenecarbonate, 4-fluoro-5-vinylethylene carbonate, 4-fluoro-5-allylethylenecarbonate, 4,4-difluoro-4-vinylethylene carbonate,4,4-difluoro-4-allylethylene carbonate, 4,5-difluoro-4-vinylethylenecarbonate, 4,5-difluoro-4-allylethylene carbonate,4-fluoro-4,5-divinylethylene carbonate, 4-fluoro-4,5-diallylethylenecarbonate, 4,5-difluoro-4,5-divinylethylene carbonate and4,5-difluoro-4,5-diallylethylene carbonate are preferable because thesefluorinated unsaturated cyclic carbonates form stable interfaceprotective films.

The molecular weight of the fluorinated unsaturated cyclic carbonates isnot particularly limited. The molecular weight is preferably 50 or moreand is preferably 250 or less. This range of molecular weights ensuresthat the fluorinated cyclic carbonate will exhibit solubility withrespect to the nonaqueous electrolytic solution and the advantageouseffects of the invention will be obtained prominently. The fluorinatedunsaturated cyclic carbonates may be produced by any methods withoutlimitation, and known production methods may be selected appropriately.The molecular weight is more preferably 100 or more, and is morepreferably 200 or less.

The fluorinated unsaturated cyclic carbonate may be used singly, or twoor more may be used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the amount of thefluorinated unsaturated cyclic carbonate (the total amount when two ormore kinds of the carbonates are used) is preferably 0.001 mass % orabove, more preferably 0.01 mass % or above, still more preferably 0.1mass % or above, and particularly preferably 0.2 mass % or above, and ispreferably 10 mass % or less, more preferably 5 mass % or less, stillmore preferably 4 mass % or less, and particularly preferably 3 mass %or less. This amount ensures that the nonaqueous electrolyte secondarybatteries will achieve a sufficient enhancement in cycle characteristicsand also reduces the risk that high-temperature storage characteristicsare decreased to cause a heavy generation of gas and a poor retention ofdischarge capacity.

In view of the formation of a composite interface protective film on thenegative electrode, the mass ratio between the aromatic carboxylateester represented by Formula (2) and the fluorine-containing cycliccarbonate is preferably 1:99 to 99:1, more preferably 5:95 to 95:5,still more preferably 10:90 to 90:10, particularly preferably 20:80 to80:20, and highly preferably 30:70 to 70:30. This ratio ensures thatside reactions of the additives on the positive and negative electrodesare suppressed efficiently, resulting in an enhancement in batterycharacteristics. In particular, this ratio of the compounds is useful inorder to reduce the initial gas production and to enhance the overchargesafety.

1-3-2. Sulfur-Containing Organic Compounds

The electrolytic solution of the invention may further include asulfur-containing organic compound. The sulfur-containing organiccompounds are not particularly limited as long as the compounds areorganic and contain at least one sulfur atom in the molecule. Thoseorganic compounds having a S═O group in the molecule are preferable,with examples including chain sulfonate esters, cyclic sulfonate esters,chain sulfate esters, cyclic sulfate esters, chain sulfite esters andcyclic sulfite esters. Fluorosulfonate salts are not categorized as thesulfur-containing organic compounds (1-3-2.) but are categorized asfluorosulfonate salt electrolytes described later.

In the electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and thesulfur-containing organic compound enhances the initial efficiency of abattery containing the electrolytic solution, and also increases theovercharge gas production in the battery to make it possible to furtherenhance the battery safety.

In particular, chain sulfonate esters, cyclic sulfonate esters, chainsulfate esters, cyclic sulfate esters, chain sulfite esters and cyclicsulfite esters are preferable, and compounds having a S(═O)₂ group aremore preferable.

These esters may have a substituent. Here, the substituent is a groupcomposed of one or more atoms selected from the group consisting ofcarbon atoms, hydrogen atoms, nitrogen atoms, oxygen atoms, sulfuratoms, phosphorus atoms and halogen atoms; preferably a group composedof one or more atoms selected from the group consisting of carbon atoms,hydrogen atoms, oxygen atoms and halogen atoms; and more preferably agroup composed of one or more atoms selected from the group consistingof carbon atoms, hydrogen atoms and oxygen atoms. Examples of thesubstituents include halogen atoms; unsubstituted or halogenated alkylgroups, alkenyl groups, alkynyl groups, aryl groups or alkoxy groups;cyano groups; isocyanate groups; alkoxycarbonyloxy groups; acyl groups;carboxyl groups; alkoxycarbonyl groups; acyloxy groups; alkylsulfonylgroups; alkoxysulfonyl groups; dialkoxyphosphanetriyl groups;dialkoxyphosphoryl groups; and dialkoxyphosphoryloxy groups. Of these,preferred substituents are halogen atoms; alkoxy groups; unsubstitutedor halogenated alkyl groups, alkenyl groups or alkynyl groups;isocyanate groups; cyano groups; alkoxycarbonyloxy groups; acyl groups;alkoxycarbonyl groups; and acyloxy groups. Halogen atoms; unsubstitutedalkyl groups; alkoxycarbonyloxy groups; acyl groups; alkoxycarbonylgroups; and acyloxy groups are more preferable. Halogen atoms,unsubstituted alkyl groups and alkoxycarbonyl groups are still morepreferable. These examples and preferred examples of the substituentsare also applied to substituents in the definitions of A¹² and A¹³ inFormula (3-2-1) and to substituents in the definition of A¹⁴ in Formula(3-2-2) described later.

Chain sulfonate esters and cyclic sulfonate esters are more preferable.In particular, chain sulfonate esters represented by Formula (3-2-1) andcyclic sulfonate esters represented by Formula (3-2-2) are preferable,and cyclic sulfonate esters represented by Formula (3-2-2) are morepreferable.

1-3-2-1. Chain Sulfonate Esters Represented by Formula (3-2-1)

(In the formula,

A¹² is an optionally substituted, n²¹-valent hydrocarbon group having 1to 12 carbon atoms,

A¹³ is an optionally substituted hydrocarbon group having 1 to 12 carbonatoms,

n²¹ is an integer of 1 to 4, and

when n²¹ is 2, A¹² and A¹³ may be the same as or different from eachother.)

In Formula (3-2-1), A¹² and A¹³ do not form a ring together, and thusthe sulfonate esters of Formula (3-2-1) are chain esters.

n²¹ is preferably an integer of 1 to 3, more preferably 1 to 2, andstill more preferably 2.

Examples of the n²¹-valent hydrocarbon groups with 1 to 12 carbon atomsrepresented by A¹² include:

monovalent hydrocarbon groups such as alkyl groups, alkenyl groups,alkynyl groups and aryl groups;

divalent hydrocarbon groups such as alkylene groups, alkenylene groups,alkynylene groups and arylene groups;

trivalent hydrocarbon groups such as alkanetriyl groups, alkenetriylgroups, alkynetriyl groups and arenetriyl groups; and

tetravalent hydrocarbon groups such as alkanetetrayl groups,alkenetetrayl groups, alkynetetrayl groups and arenetetrayl groups.

Of these, divalent hydrocarbon groups such as alkylene groups,alkenylene groups, alkynylene groups and arylene groups are preferable,and alkylene groups are more preferable. These groups correspond to theformula in which n²¹ is 2.

Of the n²-valent hydrocarbon groups having 1 to 12 carbon atoms,examples of the monovalent hydrocarbon groups include alkyl groupshaving 1 to 5 carbon atoms such as methyl group, ethyl group, n-propylgroup, i-propyl group, n-butyl group, sec-butyl group, i-butyl group,tert-butyl group, n-pentyl group, isopentyl group, sec-pentyl group,neopentyl group, 1-methylbutyl group, 2-methylbutyl group,1,1-dimethylpropyl group and 1,2-dimethylpropyl group; alkenyl groupshaving 2 to 5 carbon atoms such as vinyl group, 1-propenyl group,2-propenyl group, isopropenyl group, 1-butenyl group, 2-butenyl group,3-butenyl group, 1-pentenyl group, 2-pentenyl group, 3-pentenyl groupand 4-pentenyl group; and alkynyl groups having 2 to 5 carbon atoms suchas ethynyl group, 1-propynyl group, 2-propynyl group, 1-butynyl group,2-butynyl group, 3-butynyl group, 1-pentynyl group, 2-pentynyl group,3-pentynyl group and 4-pentynyl group.

Examples of the divalent hydrocarbon groups include alkylene groupshaving 1 to 5 carbon atoms such as methylene group, ethylene group,trimethylene group, tetramethylene group and pentamethylene group;alkenylene groups having 2 to 5 carbon atoms such as vinylene group,1-propenylene group, 2-propenylene group, 1-butenylene group,2-butenylene group, 1-pentenylene group and 2-pentenylene group; andalkynylene groups having 2 to 5 carbon atoms such as ethynylene group,propynylene group, 1-butynylene group, 2-butynylene group, 1-pentylenegroup and 2-pentylene group. Alkylene groups having 1 to 5 carbon atomssuch as methylene group, ethylene group, trimethylene group,tetramethylene group and pentamethylene group are preferable. Alkylenegroups having 2 to 5 carbon atoms such as ethylene group, trimethylenegroup, tetramethylene group and pentamethylene group are morepreferable. Alkylene groups having 3 to 5 carbon atoms such astrimethylene group, tetramethylene group and pentamethylene group arestill more preferable.

Examples of the trivalent and tetravalent hydrocarbon groups includethose trivalent and tetravalent hydrocarbon groups that correspond tothe monovalent hydrocarbon groups described above.

The substituted, n²¹-valent hydrocarbon groups with 1 to 12 carbon atomsthat are represented by A¹² are combinations of the substituentsdescribed hereinabove and the n²¹-valent hydrocarbon groups having 1 to12 carbon atoms. A¹² preferably represents an unsubstituted, n²¹-valenthydrocarbon group having 1 to 5 carbon atoms.

Preferred examples of the hydrocarbon groups with 1 to 12 carbon atomsrepresented by A¹³ include monovalent hydrocarbon groups such as alkylgroups, alkenyl groups, alkynyl groups and aryl groups, with alkylgroups being more preferable.

Examples of the hydrocarbon groups having 1 to 12 carbon atoms includealkyl groups having 1 to 5 carbon atoms such as methyl group, ethylgroup, n-propyl group, i-propyl group, n-butyl group, sec-butyl group,i-butyl group, tert-butyl group, n-pentyl group, isopentyl group,sec-pentyl group, neopentyl group, 1-methylbutyl group, 2-methylbutylgroup, 1,1-dimethylpropyl group and 1,2-dimethylpropyl group. Methylgroup, ethyl group, n-propyl group, n-butyl group and n-pentyl group arepreferable. Methyl group, ethyl group and n-propyl group are morepreferable. Ethyl group and n-propyl group are still more preferable.

The substituted hydrocarbon groups with 1 to 12 carbon atoms that arerepresented by A¹³ are combinations of the substituents describedhereinabove and the hydrocarbon groups having 1 to 12 carbon atoms. A¹³preferably represents an optionally substituted hydrocarbon group having1 to 5 carbon atoms, more preferably a substituted hydrocarbon grouphaving 1 to 5 carbon atoms, and still more preferably an alkyl grouphaving an alkoxycarbonyl group as the substituent. In particular,methoxycarbonylmethyl group, ethoxycarbonylmethyl group,1-methoxycarbonylethyl group, 1-ethoxycarbonylethyl group,2-methoxycarbonylethyl group, 2-ethoxycarbonylethyl group,1-methoxycarbonylpropyl group, 1-ethoxycarbonylpropyl group,2-methoxycarbonylpropyl group, 2-ethoxycarbonylpropyl group,3-methoxycarbonylpropyl group and 3-ethoxycarbonylpropyl group arepreferable, and 1-methoxycarbonylethyl group and 1-ethoxycarbonylethylgroup are more preferable.

In 100 mass % of the electrolytic solution, the content of the chainsulfonate ester represented by Formula (3-2-1) (the total content whentwo or more kinds of the esters are used) may be 0.001 mass % or above,preferably 0.01 mass % or above, more preferably 0.1 mass % or above,still more preferably 0.3 mass % or above, and particularly preferably0.5 mass % or above, and may be 10 mass % or less, preferably 5 mass %or less, more preferably 3 mass % or less, still more preferably 2 mass% or less, and particularly preferably 1.5 mass % or less. This contentensures good high-temperature storage characteristics.

1-3-2-2. Cyclic Sulfonate Esters Represented by Formula (3-2-2)

(In the formula,

A¹⁴ is an optionally substituted, divalent hydrocarbon group having 1 to12 carbon atoms.)

Examples of the divalent hydrocarbon groups with 1 to 12 carbon atomsrepresented by A¹⁴ include alkylene groups, alkenylene groups,alkynylene groups and arylene groups, with alkylene groups andalkenylene groups being preferable.

Examples of the divalent hydrocarbon groups having 1 to 12 carbon atomsinclude alkylene groups having 1 to 5 carbon atoms such as methylenegroup, ethylene group, trimethylene group, tetramethylene group andpentamethylene group; alkenylene groups having 2 to 5 carbon atoms suchas vinylene group, 1-propenylene group, 2-propenylene group,1-butenylene group, 2-butenylene group, 1-pentenylene group and2-pentenylene group; and

alkynylene groups having 2 to 5 carbon atoms such as ethynylene group,propynylene group, 1-butynylene group, 2-butynylene group, 1-pentylenegroup and 2-pentylene group.

Of these, preferred groups are alkylene groups having 1 to 5 carbonatoms such as methylene group, ethylene group, trimethylene group,tetramethylene group and pentamethylene group, and alkenylene groupshaving 2 to 5 carbon atoms such as vinylene group, 1-propenylene group,2-propenylene group, 1-butenylene group, 2-butenylene group,1-pentenylene group and 2-pentenylene group. More preferred groups arealkylene groups having 3 to 5 carbon atoms such as trimethylene group,tetramethylene group and pentamethylene group, and alkenylene groupshaving 3 to 5 carbon atoms such as 1-propenylene group, 2-propenylenegroup, 1-butenylene group, 2-butenylene group, 1-pentenylene group and2-pentenylene group. Trimethylene group, 1-propenylene group and2-propenylene group are still more preferable.

The substituted, divalent hydrocarbon groups with 1 to 12 carbon atomsthat are represented by A¹⁴ are combinations of the substituentsdescribed hereinabove and the divalent hydrocarbon groups having 1 to 12carbon atoms. A¹⁴ preferably represents an unsubstituted, divalenthydrocarbon group having 1 to 5 carbon atoms.

Batteries which use the electrolytic solution of the inventionadditionally including the cyclic sulfonate ester of Formula (3-2-2)exhibit an enhanced initial efficiency and an increased overcharge gasproduction to achieve a further enhancement in battery safety. In 100mass % of the electrolytic solution, the content of the cyclic sulfonateester represented by Formula (3-2-2) (the total content when two or morekinds of the esters are used) may be 0.001 mass % or above, preferably0.01 mass % or above, more preferably 0.1 mass % or above, still morepreferably 0.3 mass % or above, and particularly preferably 0.4 mass %or above, and may be 10 mass % or less, preferably 5 mass % or less,more preferably 3 mass % or less, still more preferably 2 mass % orless, and particularly preferably 1.5 mass % or less.

Examples of the sulfur-containing organic compounds include thefollowing.

Chain Sulfonate Esters

Examples include fluorosulfonate esters such as methyl fluorosulfonateand ethyl fluorosulfonate;

methanesulfonate esters such as methyl methanesulfonate, ethylmethanesulfonate, 2-propynyl methanesulfonate, 3-butynylmethanesulfonate, busulfan, methyl 2-(methanesulfonyloxy)propionate,ethyl 2-(methanesulfonyloxy)propionate, 2-propynyl2-(methanesulfonyloxy)propionate, 3-butynyl2-(methanesulfonyloxy)propionate, methyl methanesulfonyloxyacetate,ethyl methanesulfonyloxyacetate, 2-propynyl methanesulfonyloxyacetateand 3-butynyl methanesulfonyloxyacetate;

alkenylsulfonate esters such as methyl vinylsulfonate, ethylvinylsulfonate, allyl vinylsulfonate, propargyl vinylsulfonate, methylallylsulfonate, ethyl allylsulfonate, allyl allylsulfonate, propargylallylsulfonate and 1,2-bis(vinylsulfonyloxy)ethane; and

alkyldisulfonate esters such as methoxycarbonylmethylmethanedisulfonate, ethoxycarbonylmethyl methanedisulfonate,1-methoxycarbonylethyl methanedisulfonate, 1-ethoxycarbonylethylmethanedisulfonate, methoxycarbonylmethyl 1,2-ethanedisulfonate,ethoxycarbonylmethyl 1,2-ethanedisulfonate, 1-methoxycarbonylethyl1,2-ethanedisulfonate, 1-ethoxycarbonylethyl 1,2-ethanedisulfonate,methoxycarbonylmethyl 1,3-propanedisulfonate, ethoxycarbonylmethyl1,3-propanedisulfonate, 1-methoxycarbonylethyl 1,3-propanedisulfonate,1-ethoxycarbonylethyl 1,3-propanedisulfonate, methoxycarbonylmethyl1,3-butanedisulfonate, ethoxycarbonylmethyl 1,3-butanedisulfonate,1-methoxycarbonylethyl 1,3-butanedisulfonate and 1-ethoxycarbonylethyl1,3-butanedisulfonate.

Cyclic Sulfonate Esters

Examples include sultone compounds such as 1,3-propanesultone,1-fluoro-1,3-propanesultone, 2-fluoro-1,3-propanesultone,3-fluoro-1,3-propanesultone, 1-methyl-1,3-propanesultone,2-methyl-1,3-propanesultone, 3-methyl-1,3-propanesultone,1-propene-1,3-sultone, 2-propene-1,3-sultone,1-fluoro-1-propene-1,3-sultone, 2-fluoro-1-propene-1,3-sultone,3-fluoro-1-propene-1,3-sultone, 1-fluoro-2-propene-1,3-sultone,2-fluoro-2-propene-1,3-sultone, 3-fluoro-2-propene-1,3-sultone,1-methyl-1-propene-1,3-sultone, 2-methyl-1-propene-1,3-sultone,3-methyl-1-propene-1,3-sultone, 1-methyl-2-propene-1,3-sultone,2-methyl-2-propene-1,3-sultone, 3-methyl-2-propene-1,3-sultone,1,4-butanesultone and 1,5-pentanesultone;

disulfonate compounds such as methylene methanedisulfonate and ethylenemethanedisulfonate;

nitrogen-containing compounds such as 1,2,3-oxathiazolidine-2,2-dioxide,3-methyl-1,2,3-oxathiazolidine-2,2-dioxide,3H-1,2,3-oxathiazole-2,2-dioxide, 5H-1,2,3-oxathiazole-2,2-dioxide,1,2,4-oxathiazolidine-2,2-dioxide, 1,2,5-oxathiazolidine-2,2-dioxide,1,2,3-oxathiazinane-2,2-dioxide,3-methyl-1,2,3-oxathiazinane-2,2-dioxide,5,6-dihydro-1,2,3-oxathiazine-2,2-dioxide and1,2,4-oxathiazinane-2,2-dioxide; and

phosphorus-containing compounds such as1,2,3-oxathiaphosphorane-2,2-dioxide,3-methyl-1,2,3-oxathiaphosphorane-2,2-dioxide,3-methyl-1,2,3-oxathiaphosphorane-2,2,3-trioxide,3-methoxy-1,2,3-oxathiaphosphorane-2,2,3-trioxide,1,2,4-oxathiaphosphorane-2,2-dioxide,1,2,5-oxathiaphosphorane-2,2-dioxide,1,2,3-oxathiaphosphinane-2,2-dioxide,3-methyl-1,2,3-oxathiaphosphinane-2,2-dioxide,3-methyl-1,2,3-oxathiaphosphinane-2,2,3-trioxide,3-methoxy-1,2,3-oxathiaphosphinane-2,2,3-trioxide,1,2,4-oxathiaphosphinane-2,2-dioxide,1,2,5-oxathiaphosphinane-2,2-dioxide and1,2,6-oxathiaphosphinane-2,2-dioxide.

Chain Sulfate Esters

Examples include dialkyl sulfate compounds such as dimethyl sulfate,ethylmethyl sulfate and diethyl sulfate.

Cyclic Sulfate Ester

Examples include alkylene sulfate compounds such as 1,2-ethylenesulfate, 1,2-propylene sulfate, 1,3-propylene sulfate, 1,2-butylenesulfate, 1,3-butylene sulfate, 1,4-butylene sulfate, 1,2-pentylenesulfate, 1,3-pentylene sulfate, 1,4-pentylene sulfate and 1,5-pentylenesulfate.

Chain Sulfite Esters

Examples include dialkyl sulfite compounds such as dimethyl sulfite,ethylmethyl sulfite and diethyl sulfite.

Cyclic Sulfite Ester

Examples include alkylene sulfite compounds such as 1,2-ethylenesulfite, 1,2-propylene sulfite, 1,3-propylene sulfite, 1,2-butylenesulfite, 1,3-butylene sulfite, 1,4-butylene sulfite, 1,2-pentylenesulfite, 1,3-pentylene sulfite, 1,4-pentylene sulfite and 1,5-pentylenesulfite.

Of these, methyl 2-(methanesulfonyloxy)propionate, ethyl2-(methanesulfonyloxy)propionate, 2-propynyl2-(methanesulfonyloxy)propionate, 1-methoxycarbonylethylpropanedisulfonate, 1-ethoxycarbonylethyl propanedisulfonate,1-methoxycarbonylethyl butanedisulfonate, 1-ethoxycarbonylethylbutanedisulfonate, 1,3-propanesultone, 1-propene-1,3-sultone,1,4-butanesultone, 1,2-ethylene sulfate, 1,2-ethylene sulfite, methylmethanesulfonate and ethyl methanesulfonate are preferable from thepoint of view of enhancing the initial efficiency. More preferredcompounds are 1-methoxycarbonylethyl propanedisulfonate,1-ethoxycarbonylethyl propanedisulfonate, 1-methoxycarbonylethylbutanedisulfonate, 1-ethoxycarbonylethyl butanedisulfonate,1,3-propanesultone, 1-propene-1,3-sultone, 1,2-ethylene sulfate and1,2-ethylene sulfite. 1,3-Propanesultone and 1-propene-1,3-sultone arestill more preferable.

The sulfur-containing organic compounds may be used singly, or two ormore may be used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the content of thesulfur-containing organic compound (the total content when two or morekinds of the compounds are used) may be 0.001 mass % or above,preferably 0.01 mass % or above, more preferably 0.1 mass % or above,and particularly preferably 0.3 mass % or above, and may be 10 mass % orless, preferably 5 mass % or less, more preferably 3 mass % or less, andparticularly preferably 2 mass % or less. This content ensures easycontrol of characteristics such as output characteristics, loadcharacteristics, low-temperature characteristics, cycle characteristicsand high-temperature storage characteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the sulfur-containing organic compound, namely, thearomatic carboxylate ester of Formula (2):sulfur-containing organiccompound ratio is preferably 1:99 to 99:1, more preferably 5:95 to 95:5,still more preferably 10:90 to 90:10, particularly preferably 20:80 to80:20, and highly preferably 30:70 to 70:30. This ratio ensures thatbattery characteristics, in particular, initial characteristics will bemarkedly enhanced. Although the mechanism of this effect is not clear,it is probable that the additives mixed in the above ratio are preventedfrom side reactions on the electrodes to the maximum extent.

1-3-3. Phosphonate Esters

The electrolytic solution of the invention may further include aphosphonate ester. The phosphonate esters are not particularly limitedas long as the compounds are organic and contain at least a phosphonateester structure in the molecule.

In the electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and thephosphonate ester enhances the initial rate characteristic of a batterycontaining the electrolytic solution, and also enhances the batterycapacity after storage.

The phosphonate esters may have a substituent. Here, the substituent isa group composed of one or more atoms selected from the group consistingof carbon atoms, hydrogen atoms, nitrogen atoms, oxygen atoms, sulfuratoms, phosphorus atoms and halogen atoms; and preferably a groupcomposed of one or more atoms selected from the group consisting ofcarbon atoms, hydrogen atoms, oxygen atoms and halogen atoms. Examplesof the substituents include halogen atoms; unsubstituted or halogenatedalkyl groups, alkenyl groups, alkynyl groups, aryl groups or alkoxygroups; cyano groups; isocyanate groups; alkoxycarbonyloxy groups; acylgroups; alkoxycarbonyl groups; acyloxy groups; alkylsulfonyl groups;alkoxysulfonyl groups; dialkoxyphosphanetriyl groups; dialkoxyphosphorylgroups; and dialkoxyphosphoryloxy groups. Of these, for example,preferred substituents are halogen atoms; alkoxy groups;alkoxycarbonyloxy groups; acyl groups; alkoxycarbonyl groups; andacyloxy groups. Halogen atoms and alkoxycarbonyl groups are morepreferable, and alkoxycarbonyl groups are still more preferable.Examples of the alkoxycarbonyl groups include methoxycarbonyl group,ethoxycarbonyl group, propoxycarbonyl group, allyloxycarbonyl group andpropargyloxycarbonyl group, with ethoxycarbonyl group andpropargyloxycarbonyl group being preferable. These examples andpreferred examples of the substituents are also applied to substituentsin the definitions of A⁹ to A¹¹ in Formula (3-3-1) described below.

In particular, phosphonate esters represented by Formula (3-3-1) arepreferable.

(In the formula,

A⁹, A¹⁰ and A¹¹ are independently an unsubstituted or halogenated alkyl,alkenyl or alkynyl group having 1 to 5 carbon atoms, and

n³² is an integer of 0 to 6.)

Examples of the alkyl, alkenyl or alkynyl groups having 1 to 5 carbonatoms include alkyl groups such as methyl group, ethyl group, n-propylgroup, i-propyl group, n-butyl group, sec-butyl group, i-butyl group,tert-butyl group, n-pentyl group, isopentyl group, sec-pentyl group,neopentyl group, 1-methylbutyl group, 2-methylbutyl group,1,1-dimethylpropyl group and 1,2-dimethylpropyl group; alkenyl groupssuch as vinyl group, 1-propenyl group, 2-propenyl group (allyl group),isopropenyl group, 1-butenyl group, 2-butenyl group, 3-butenyl group,1-pentenyl group, 2-pentenyl group, 3-pentenyl group and 4-pentenylgroup; and alkynyl groups such as ethynyl group, 1-propynyl group,2-propynyl group (propargyl group), 1-butynyl group, 2-butynyl group,3-butynyl group, 1-pentynyl group, 2-pentynyl group, 3-pentynyl groupand 4-pentynyl group. Preferred groups are methyl group, ethyl group,n-propyl group, n-butyl group, n-pentyl group, vinyl group, 2-propenylgroup (allyl group), 3-butenyl group, 4-pentenyl group, 2-propynyl group(propargyl group), 3-butynyl group and 4-pentynyl group. Methyl group,ethyl group, 2-propenyl group (allyl group) and 2-propynyl group(propargyl group) are more preferable, and methyl group, ethyl group and2-propynyl group (propargyl group) are still more preferable.

Examples of the phosphonate esters represented by Formula (3-3-1)include the following compounds.

Compounds of Formula (3-3-1) in which n³²=0

Examples include trimethyl phosphonoformate, methyldiethylphosphonoformate, methyl dipropylphosphonoformate, methyldibutylphosphonoformate, triethyl phosphonoformate, ethyldimethylphosphonoformate, ethyl dipropylphosphonoformate, ethyldibutylphosphonoformate, tripropyl phosphonoformate, propyldimethylphosphonoformate, propyl diethylphosphonoformate, propyldibutylphosphonoformate, tributyl phosphonofonrmate, butyldimethylphosphonoformate, butyl diethylphosphonoformate, butyldipropylphosphonoformate, methylbis(2,2,2-trifluoroethyl)phosphonoformate, ethylbis(2,2,2-trifluoroethyl)phosphonoformate, propylbis(2,2,2-trifluoroethyl)phosphonoformate and butylbis(2,2,2-trifluoroethyl)phosphonoformate.

Compounds of Formula (3-3-1) in which n³²=1

Examples include trimethyl phosphonoacetate, methyldiethylphosphonoacetate, methyl dipropylphosphonoacetate, methyldibutylphosphonoacetate, triethyl phosphonoacetate, ethyldimethylphosphonoacetate, ethyl diethylphosphonoacetate, ethyldipropylphosphonoacetate, ethyl dibutylphosphonoacetate, tripropylphosphonoacetate, propyl dimethylphosphonoacetate, propyldiethylphosphonoacetate, propyl dibutylphosphonoacetate, tributylphosphonoacetate, butyl dimethylphosphonoacetate, butyldiethylphosphonoacetate, butyl dipropylphosphonoacetate, methylbis(2,2,2-trifluoroethyl)phosphonoacetate, ethylbis(2,2,2-trifluoroethyl)phosphonoacetate, propylbis(2,2,2-trifluoroethyl)phosphonoacetate, butylbis(2,2,2-trifluoroethyl)phosphonoacetate, allyldimethylphosphonoacetate, allyl diethylphosphonoacetate, 2-propynyldimethylphosphonoacetate and 2-propynyl diethylphoshonoacetate.

Compounds of Formula (3-3-1) in which n³²=2

Examples include trimethyl 3-phosphonopropionate, methyl3-(diethylphosphono)propionate, methyl 3-(dipropylphosphono)propionate,methyl 3-(dibutylphosphono)propionate, triethyl 3-phosphonopropionate,ethyl 3-(dimethylphosphono)propionate, ethyl3-(dipropylphosphono)propionate, ethyl 3-(dibutylphosphono)propionate,tripropyl 3-phosphonopropionate, propyl 3-(dimethylphosphono)propionate,propyl 3-(diethylphosphono)propionate, propyl3-(dibutylphosphono)propionate, tributyl 3-phosphonopropionate, butyl3-(dimethylphosphono)propionate, butyl 3-(diethylphosphono)propionate,butyl 3-(dipropylphosphono)propionate, methyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, ethyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, propyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate and butyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate.

Compounds of Formula (3-3-1) in which n³²=3

Examples include trimethyl 4-phosphonobutyrate, methyl4-(diethylphosphono)butyrate, methyl 4-(dipropylphosphono)butyrate,methyl 4-(dibutylphosphono)butyrate, triethyl 4-phosphonobutyrate, ethyl4-(dimethylphosphono)butyrate, ethyl 4-(dipropylphosphono)butyrate,ethyl 4-(dibutylphosphono)butyrate, tripropyl 4-phosphonobutyrate,propyl 4-(dimethylphosphono)butyrate, propyl4-(diethylphosphono)butyrate, propyl 4-(dibutylphosphono)butyrate,tributyl 4-phosphonobutyrate, butyl 4-(dimethylphosphono)butyrate, butyl4-(diethylphosphono)butyrate and butyl 4-(dipropylphosphono)butyrate.

From the point of view of enhancing battery characteristics, thosecompounds in which n³²=0, 1 or 2 are preferable, those compounds inwhich n³²=0 or 1 are more preferable, and those compounds in which n³²=1are still more preferable. Of the compounds in which n³²=1, thosecompounds in which A⁹ to A¹¹ are saturated hydrocarbon groups arepreferable.

In particular, trimethyl phosphonoacetate, triethyl phosphonoacetate,2-propynyl dimethylphosphonoacetate and 2-propynyldiethylphosphonoacetate are preferable.

The phosphonate esters may be used singly, or two or more may be used incombination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the amount of thephosphonate ester (the total amount when two or more kinds of the estersare used) may be 0.001 mass % or above, preferably 0.01 mass % or above,more preferably 0.1 mass % or above, and still more preferably 0.4 mass% or above, and may be 10 mass % or less, preferably 5 mass % or less,more preferably 3 mass % or less, still more preferably 2 mass % orless, particularly preferably 1 mass % or less, and most preferably 0.7mass % or less. This amount ensures easy control of characteristics suchas output characteristics, load characteristics, low-temperaturecharacteristics, cycle characteristics and high-temperature storagecharacteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the phosphonate ester, namely, the aromatic carboxylateester of Formula (2):phosphonate ester ratio is preferably 1:99 to 99:1,more preferably 5:95 to 95:5, still more preferably 10:90 to 90:10,particularly preferably 20:80 to 80:20, and highly preferably 30:70 to70:30. This ratio ensures that battery characteristics, in particular,initial characteristics will be markedly enhanced. Although themechanism of this effect is not clear, it is probable that the additivesmixed in the above ratio are prevented from side reactions on theelectrodes to the maximum extent.

1-3-4. Cyano Group-Containing Organic Compounds

The electrolytic solution of the invention may further include a cyanogroup-containing organic compound. The cyano group-containing organiccompounds are not particularly limited as long as the compounds areorganic and have at least one cyano group in the molecule. Thosecompounds represented by Formulae (3-4-1), (3-4-2) and (3-4-3) arepreferable. Those compounds represented by Formulae (3-4-1) and (3-4-2)are more preferable. Those compounds represented by Formula (3-4-2) arestill more preferable. Those cyano group-containing organic compoundswhich are cyclic compounds having a plurality of ether bonds arecategorized into the cyclic compounds having a plurality of ether bonds.

In the electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and the cyanogroup-containing organic compound enhances the initial charge dischargeefficiency of a battery containing the electrolytic solution, and alsoenhances the charge discharge efficiency after storage of the battery.

1-3-4-1. Compounds Represented by Formula (3-4-1)

A¹-CN  (3-4-1)

(In the formula, A¹ is a hydrocarbon group having 2 to 20 carbon atoms.)

The molecular weight of the compounds represented by Formula (3-4-1) isnot particularly limited. The molecular weight is preferably 55 or more,more preferably 65 or more, and still more preferably 80 or more, and ispreferably 310 or less, more preferably 185 or less, and still morepreferably 155 or less. This range of molecular weights ensures that thecompound of Formula (3-4-1) will exhibit solubility with respect to thenonaqueous electrolytic solution and the advantageous effects of theinvention will be obtained prominently. The compounds of Formula (3-4-1)may be produced by any methods without limitation, and known productionmethods may be selected appropriately.

Referring to Formula (3-4-1), examples of the hydrocarbon groups having2 to 20 carbon atoms include alkyl groups, alkenyl groups, alkynylgroups and aryl groups. Preferred examples include alkyl groups such asethyl group, n-propyl group, iso-propyl group, n-butyl group, sec-butylgroup, iso-butyl group, tert-butyl group, n-pentyl group, tert-amylgroup, hexyl group, heptyl group, octyl group, nonyl group, decyl group,undecyl group, dodecyl group, tridecyl group, tetradecyl group,pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group,nonadecyl group and eicosyl group; alkenyl groups such as vinyl group,1-propenyl group, isopropenyl group, 1-butenyl group and 1-pentenylgroup; alkynyl groups such as ethynyl group, 1-propynyl group, 1-butynylgroup and 1-pentynyl group; and aryl groups such as phenyl group, tolylgroup, ethylphenyl group, n-propylphenyl group, i-propylphenyl group,n-butylphenyl group, sec-butylphenyl group, i-butylphenyl group,tert-butylphenyl group, trifluoromethylphenyl group, xylyl group, benzylgroup, phenethyl group, methoxyphenyl group, ethoxyphenyl group andtrifluoromethoxyphenyl group.

In particular, linear or branched alkyl groups having 2 to 15 carbonatoms and alkenyl groups having 2 to 4 carbon atoms are more preferable,linear or branched alkyl groups having 2 to 12 carbon atoms are stillmore preferable, and linear or branched alkyl groups having 4 to 11carbon atoms are particularly preferable in view of the facts that suchcompounds have the cyano groups in a high proportion relative to theentirety of the molecule and provide high effects in the enhancement ofbattery characteristics.

Examples of the compounds represented by Formula (3-4-1) includepropionitrile, butyronitrile, pentanenitrile, hexanenitrile,heptanenitrile, octanenitrile, pelargononitrile, decanenitrile,undecanenitrile, dodecanenitrile, cyclopentanecarbonitrile,cyclohexanecarbonitrile, acrylonitrile, methacrylonitrile,crotononitrile, 3-methylcrotononitrile, 2-methyl-2-butenenitrile,2-pentenenitrile, 2-methyl-2-pentenenitrile, 3-methyl-2-pentenenitrileand 2-hexenenitrile.

From the points of view of the stability of the compounds, batterycharacteristics and production perspectives, pentanenitrile,octanenitrile, decanenitrile, dodecanenitrile and crotononitrile arepreferable, pentanenitrile, decanenitrile, dodecanenitrile andcrotononitrile are more preferable, and pentanenitrile, decanenitrileand crotononitrile are preferable.

The compounds of Formula (3-4-1) may be used singly, or two or more maybe used in combination in an appropriate ratio. In 100 mass % of thenonaqueous electrolytic solution, the amount of the compound representedby Formula (3-4-1) (the total amount when two or more kinds of thecompounds are used) may be 0.001 mass % or above, preferably 0.01 mass %or above, and more preferably 0.1 mass % or above, and may be 10 mass %or less, preferably 5 mass % or less, more preferably 3 mass % or less,still more preferably 2 mass % or less, particularly preferably 1 mass %or less, and most preferably 0.5 mass % or less. This content ensureseasy control of characteristics such as output characteristics, loadcharacteristics, low-temperature characteristics, cycle characteristicsand high-temperature storage characteristics.

1-3-4-2. Compounds Represented by Formula (3-4-2)

NC-A²-CN  (3-4-2)

(In the formula,

A² is an organic group with 1 to 10 carbon atoms that is composed of oneor more kinds of atoms selected from the group consisting of hydrogenatoms, carbon atoms, nitrogen atoms, oxygen atoms, sulfur atoms,phosphorus atoms and halogen atoms.)

The organic groups with 1 to 10 carbon atoms that are composed of one ormore kinds of atoms selected from the group consisting of hydrogenatoms, carbon atoms, nitrogen atoms, oxygen atoms, sulfur atoms,phosphorus atoms and halogen atoms include not only those organic groupswhich are composed of carbon and hydrogen atoms, but also those organicgroups which optionally further contain nitrogen, oxygen, sulfur,phosphorus or halogen atoms. Those organic groups which optionallyfurther contain nitrogen, oxygen, sulfur, phosphorus or halogen atomsinclude those organic groups in which the carbon atoms in thehydrocarbon skeleton are partially substituted by such atoms, and thoseorganic groups which have a substituent composed of such atoms.

The molecular weight of the compounds represented by Formula (3-4-2) isnot particularly limited. The molecular weight is preferably 65 or more,more preferably 80 or more, and still more preferably 90 or more, and ispreferably 270 or less, more preferably 160 or less, and still morepreferably 135 or less. This range of molecular weights ensures that thecompound of Formula (3-4-2) will exhibit solubility with respect to thenonaqueous electrolytic solution and the advantageous effects of theinvention will be obtained prominently. The compounds of Formula (3-4-2)may be produced by any methods without limitation, and known productionmethods may be selected appropriately.

Examples of A² in the compounds of Formula (3-4-2) include alkylenegroups and derivatives thereof, alkenylene groups and derivativesthereof, cycloalkylene groups and derivatives thereof, alkynylene groupsand derivatives thereof, cycloalkenylene groups and derivatives thereof,arylene groups and derivatives thereof, carbonyl groups and derivativesthereof, sulfonyl groups and derivatives thereof, sulfinyl groups andderivatives thereof, phosphonyl groups and derivatives thereof,phosphinyl groups and derivatives thereof, amide groups and derivativesthereof, imide groups and derivatives thereof, ether groups andderivatives thereof, thioether groups and derivatives thereof, borinategroups and derivatives thereof, and borane groups and derivativesthereof.

From the point of view of enhancing battery characteristics, preferredgroups are alkylene groups and derivatives thereof, alkenylene groupsand derivatives thereof, cycloalkylene groups and derivatives thereof,alkynylene groups and derivatives thereof, and arylene groups andderivatives thereof. More preferably, A² is an optionally substitutedalkylene group having 2 to 5 carbon atoms.

Examples of the compounds represented by Formula (3-4-2) includemalononitrile, succinonitrile, glutaronitrile, adiponitrile,pimelonitrile, suberonitrile, azelanitrile, sebaconitrile,undecanedinitrile, dodecanedinitrile, methylmalononitrile,ethylmalononitrile, isopropylmalononitrile, tert-butylmalononitrile,methylsuccinonitrile, 2,2-dimethylsuccinonitrile,2,3-dimethylsuccinonitrile, 2,3,3-trimethylsuccinonitrile,2,2,3,3-tetramethylsuccinonitrile,2,3-diethyl-2,3-dimethylsuccinonitrile,2,2-diethyl-3,3-dimethylsuccinonitrile, bicyclohexyl-1,1-dicarbonitrile,bicyclohexyl-2,2-dicarbonitrile, bicyclohexyl-3,3-dicarbonitrile,2,5-dimethyl-2,5-hexanedicarbonitrile,2,3-diisobutyl-2,3-dimethylsuccinonitrile,2,2-diisobutyl-3,3-dimethylsuccinonitrile, 2-methylglutaronitrile,2,3-dimethylglutaronitrile, 2,4-dimethylglutaronitrile,2,2,3,3-tetramethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile,2,2,3,4-tetramethylglutaronitrile, 2,3,3,4-tetramethylglutaronitrile,maleonitrile, fumaronitrile, 1,4-dicyanopentane, 2,6-dicyanoheptane,2,7-dicyanooctane, 2,8-dicyanononane, 1,6-dicyanodecane,1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene,3,3′-(ethylenedioxy)dipropionitrile,3,3′-(ethylenedithio)dipropionitrile and3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane.

Of these, malononitrile, succinonitrile, glutaronitrile, adiponitrile,pimelonitrile, suberonitrile, azelanitrile, sebaconitrile,undecanedinitrile, dodecanedinitrile,3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane andfumaronitrile are preferable from the point of view of enhancing storagecharacteristics. Further, succinonitrile, glutaronitrile, adiponitrile,pimelonitrile, suberonitrile and3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane are morepreferable because these compounds have particularly high effects in theenhancement of storage characteristics and are less prone to degradationby side reactions on the electrodes. In the dinitrile compounds, theproportion of the cyano groups in the molecule is usually increased withdecreasing molecular weight and consequently the molecules exhibit ahigher viscosity, whilst the increase in molecular weight increases theboiling point of the compounds. Thus, succinonitrile, glutaronitrile,adiponitrile and pimelonitrile are more preferable from the point ofview of enhancing the work efficiency.

The compounds of Formula (3-4-2) may be used singly, or two or more maybe used in combination in an appropriate ratio. In 100 mass % of theelectrolytic solution, the concentration of the compound represented byFormula (3-4-2) (the total concentration when two or more kinds of thecompounds are used) may be 0.001 mass % or above, preferably 0.01 mass %or above, more preferably 0.1 mass % or above, and particularlypreferably 0.3 mass % or above, and may be 10 mass % or less, preferably5 mass % or less, and more preferably 3 mass % or less. The satisfactionof this concentration increases the effects in the enhancements ofcharacteristics such as output characteristics, load characteristics,low-temperature characteristics, cycle characteristics andhigh-temperature storage characteristics.

1-3-4-3. Compounds Represented by Formula (3-4-3)

(In the formula,

A³ is an organic group with 1 to 12 carbon atoms that is composed of oneor more kinds of atoms selected from the group consisting of hydrogenatoms, carbon atoms, nitrogen atoms, oxygen atoms, sulfur atoms,phosphorus atoms and halogen atoms, and n⁴³ is an integer of 0 to 5.)

The organic groups with 1 to 12 carbon atoms that are composed of one ormore kinds of atoms selected from the group consisting of hydrogenatoms, carbon atoms, nitrogen atoms, oxygen atoms, sulfur atoms,phosphorus atoms and halogen atoms include not only those organic groupswhich are composed of carbon and hydrogen atoms, but also those organicgroups which optionally further contain nitrogen, oxygen, sulfur,phosphorus or halogen atoms. Those organic groups which optionallyfurther contain nitrogen, oxygen, sulfur, phosphorus or halogen atomsinclude those organic groups in which the carbon atoms in thehydrocarbon skeleton are partially substituted by such atoms, and thoseorganic groups which have a substituent composed of such atoms.

n⁴³ is an integer of 0 to 5, preferably 0 to 3, more preferably 0 to 1,and particularly preferably 0.

Preferably, A³ is an organic group with 1 to 12 carbon atoms that iscomposed of one or more kinds of atoms selected from the groupconsisting of hydrogen atoms, carbon atoms, nitrogen atoms, oxygen atomsand sulfur atoms. More preferably, A³ is an organic group with 1 to 12carbon atoms that is composed of one or more kinds of atoms selectedfrom the group consisting of hydrogen atoms, carbon atoms and oxygenatoms. Still more preferably, A³ is an optionally substituted aliphatichydrocarbon group having 1 to 12 carbon atoms.

Here, the substituent is a group composed of one or more atoms selectedfrom the group consisting of carbon atoms, hydrogen atoms, nitrogenatoms, oxygen atoms, sulfur atoms, phosphorus atoms and halogen atoms.

Examples of the substituents include halogen atoms; unsubstituted orhalogenated alkyl groups, alkenyl groups, alkynyl groups, aryl groups oralkoxy groups; isocyanate groups; alkoxycarbonyloxy groups; acyl groups;carboxyl groups; alkoxycarbonyl groups; acyloxy groups; alkylsulfonylgroups; alkoxysulfonyl groups; dialkoxyphosphanetriyl groups;dialkoxyphosphoryl groups; and dialkoxyphosphoryloxy groups. Preferredsubstituents are halogen atoms; alkoxy groups; and unsubstituted orhalogenated alkyl groups. Halogen atoms, and unsubstituted orhalogenated alkyl groups are more preferable. Unsubstituted alkyl groupsare still more preferable.

The aliphatic hydrocarbon groups are not particularly limited. Thenumber of carbon atoms in the groups may be 1 or more, preferably 2 ormore, and more preferably 3 or more, and may be 12 or less, preferably 8or less, and more preferably 6 or less.

Examples of the aliphatic hydrocarbon groups, in accordance with n⁴³,include alkanetriyl groups, alkanetetrayl groups, alkanepentayl groups,alkanetetrayl groups, alkenetriyl groups, alkenetetrayl groups,alkenepentayl groups, alkenetetrayl groups, alkynetriyl groups,alkynetetrayl groups, alkynepentayl groups and alkynetetrayl groups.

Of these, saturated hydrocarbon groups such as alkanetriyl groups,alkanetetrayl groups, alkanepentayl groups and alkanetetrayl groups aremore preferable, and alkanetriyl groups are still more preferable.

Further, the compounds represented by Formula (3-4-3) are morepreferably represented by Formula (3-4-3′).

(In the formula, A⁴ and A⁵ are each a divalent group corresponding toA³.)

More preferably, A⁴ and A⁵ are optionally substituted hydrocarbon groupshaving 1 to 5 carbon atoms.

Examples of the hydrocarbon groups include methylene group, ethylenegroup, trimethylene group, tetraethylene group, pentamethylene group,vinylene group, 1-propenylene group, 2-propenylene group, 1-butenylenegroup, 2-butenylene group, 1-pentenylene group, 2-pentenylene group,ethynylene group, propynylene group, 1-butynylene group, 2-butynylenegroup, 1-pentylene group and 2-pentylene group.

Of these, methylene group, ethylene group, trimethylene group,tetraethylene group and pentamethylene group are preferable, andmethylene group, ethylene group and trimethylene group are morepreferable.

It is preferable that A⁴ and A⁵ be not the same and differ from eachother.

The molecular weight of the compounds represented by Formula (3-4-3) isnot particularly limited. The molecular weight is preferably 90 or more,more preferably 120 or more, and still more preferably 150 or more, andis preferably 450 or less, more preferably 300 or less, and still morepreferably 250 or less. This range of molecular weights ensures that thecompound of Formula (3-4-3) will exhibit solubility with respect to thenonaqueous electrolytic solution and the advantageous effects of theinvention will be obtained prominently. The compounds of Formula (3-4-3)may be produced by any methods without limitation, and known productionmethods may be selected appropriately.

Examples of the compounds represented by Formula (3-4-3) include thefollowing compounds:

Of these, the following compounds are preferable from the point of viewof enhancing storage characteristics.

The cyano group-containing organic compounds may be used singly, or twoor more may be used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the concentration of thecompound represented by Formula (3-4-3) (the total concentration whentwo or more kinds of the compounds are used) may be 0.001 mass % orabove, preferably 0.01 mass % or above, more preferably 0.1 mass % orabove, and particularly preferably 0.3 mass % or above, and may be 10mass % or less, preferably 5 mass % or less, more preferably 3 mass % orless, and particularly preferably 2 mass % or less. This concentrationensures easy control of characteristics such as output characteristics,load characteristics, low-temperature characteristics, cyclecharacteristics and high-temperature storage characteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the cyano group-containing organic compound, namely, thearomatic carboxylate ester of Formula (2):cyano group-containing organiccompound ratio is preferably 1:99 to 99:1, more preferably 5:95 to 95:5,still more preferably 10:90 to 90:10, particularly preferably 20:80 to80:20, and highly preferably 30:70 to 70:30. This ratio ensures thatbattery characteristics, in particular, initial characteristics will bemarkedly enhanced. Although the mechanism of this effect is not clear,it is probable that the additives mixed in the above ratio are preventedfrom side reactions on the electrodes to the maximum extent.

1-3-5. Isocyanate Group-Containing Organic Compounds

The electrolytic solution of the invention may further include anisocyanate group-containing organic compound. The isocyanategroup-containing organic compounds are not particularly limited as longas the compounds are organic and contain at least one isocyanate groupin the molecule. The number of the isocyanate groups in the molecule ispreferably 1 to 4, more preferably 2 to 3, and still more preferably 2.

In the electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and the isocyanategroup-containing compound reduces the initial gas production in abattery containing the electrolytic solution, and also enhances thebattery capacity after storage.

The isocyanate group-containing organic compounds are preferably suchthat the isocyanate groups are bonded to compounds having a linear orbranched alkylene group, a cycloalkylene group, a structure in which acycloalkylene group and an alkylene group are linked together, anaromatic hydrocarbon group, a structure in which an aromatic hydrocarbongroup and an alkylene group are linked together, an ether structure(—O—), a structure in which an ether structure (—O—) and an alkylenegroup are linked together, a carbonyl group (—C(═O)—), a structure inwhich a carbonyl group and an alkylene group are linked together, asulfonyl group (—S(═O)—), a structure in which a sulfonyl group and analkylene group are linked together, or a structure resulting from thehalogenation of any of the groups and structures described above. Theisocyanate group-containing organic compounds are more preferably suchthat the isocyanate groups are bonded to a linear or branched alkylenegroup, a cycloalkylene group, a structure in which a cycloalkylene groupand an alkylene group are linked together, an aromatic hydrocarbongroup, or a structure in which an aromatic hydrocarbon group and analkylene group are linked together, and are still more preferably suchthat the isocyanate groups are bonded to a structure in which acycloalkylene group and an alkylene group are linked together. Themolecular weight of the isocyanate group-containing organic compounds isnot particularly limited. The molecular weight is preferably 80 or more,more preferably 115 or more, and still more preferably 170 or more, andis preferably 300 or less, and more preferably 230 or less. This rangeof molecular weights ensures that the isocyanate group-containingorganic compound will exhibit solubility with respect to the nonaqueouselectrolytic solution and the advantageous effects of the invention willbe obtained prominently. The isocyanate group-containing organiccompounds may be produced by any methods without limitation, and knownproduction methods may be selected appropriately. Further, commercialproducts may be used.

Examples of the isocyanate group-containing organic compounds includeorganic compounds having one isocyanate group such as methyl isocyanate,ethyl isocyanate, propyl isocyanate, isopropyl isocyanate, butylisocyanate, tert-butyl isocyanate, pentyl isocyanate, hexyl isocyanate,cyclohexyl isocyanate, vinyl isocyanate, allyl isocyanate, ethynylisocyanate, propargyl isocyanate, phenyl isocyanate and fluorophenylisocyanate; and

organic compounds having two isocyanate groups such as monomethylenediisocyanate, dimethylene diisocyanate, trimethylene diisocyanate,tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylenediisocyanate, heptamethylene diisocyanate, octamethylene diisocyanate,nonamethylene diisocyanate, decamethylene diisocyanate, dodecamethylenediisocyanate, 1,3-diisocyanatopropane, 1,4-diisocyanato-2-butene,1,4-diisocyanato-2-fluorobutane, 1,4-diisocyanato-2,3-difluorobutane,1,5-diisocyanato-2-pentene, 1,5-diisocyanato-2-methylpentane,1,6-diisocyanato-2-hexene, 1,6-diisocyanato-3-hexene,1,6-diisocyanato-3-fluorohexane, 1,6-diisocyanato-3,4-difluorohexane,toluene diisocyanate, xylene diisocyanate, tolylene diisocyanate,1,2-bis(isocyanatomethyl)cyclohexane,1,3-bis(isocyanatomethyl)cyclohexane,1,4-bis(isocyanatomethyl)cyclohexane, 1,2-diisocyanatocyclohexane,1,3-diisocyanatocyclohexane, 1,4-diisocyanatocyclohexane,dicyclohexylmethane-1,1′-diisocyanate,dicyclohexylmethane-2,2′-diisocyanate,dicyclohexylmethane-3,3′-diisocyanate,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl isocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate), isophoronediisocyanate, carbonyl diisocyanate, 1,4-diisocyanatobutane-1,4-dione,1,5-diisocyanatopentane-1,5-dione, 2,2,4-trimethylhexamethylenediisocyanate and 2,4,4-trimethylhexamethylene diisocyanate.

Of these, those organic compounds having two isocyanate groups arepreferable from the point of view of enhancing storage characteristics,with specific examples including monomethylene diisocyanate, dimethylenediisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate,pentamethylene diisocyanate, hexamethylene diisocyanate, heptamethylenediisocyanate, octamethylene diisocyanate, nonamethylene diisocyanate,decamethylene diisocyanate, dodecamethylene diisocyanate,1,3-bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl isocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate), isophoronediisocyanate, 2,2,4-trimethylhexamethylene diisocyanate and2,4,4-trimethylhexamethylene diisocyanate. More preferred compounds arehexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl isocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate), isophoronediisocyanate, 2,2,4-trimethylhexamethylene diisocyanate and2,4,4-trimethylhexamethylene diisocyanate.1,3-Bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl isocyanate) andbicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate) are still morepreferable.

The isocyanate group-containing organic compounds may be trimercompounds that are derived from compounds having at least two isocyanategroups in the molecule, or may be aliphatic polyisocyanates that areadducts of the trimer compounds with polyhydric alcohols. Examplesinclude biurets, isocyanurates, adducts and bifunctional modifiedpolyisocyanates having the basic structures represented by Formulae(3-5-1) to (3-5-4) below.

(In the formula, R⁵¹ to R⁵⁴ and R⁵⁴′ independently at each occurrenceare a divalent hydrocarbon group (for example, a tetramethylene group ora hexamethylene group), and R⁵³′ independently at each occurrence is atrivalent hydrocarbon group.)

The organic compounds having at least two isocyanate groups in themolecule include so-called blocked isocyanates in which functionalgroups are blocked with a blocking agent to increase storage stability.Examples of the blocking agents include alcohols, phenols, organicamines, oximes and lactams. Specific examples include n-butanol, phenol,tributylamine, diethylethanolamine, methyl ethyl ketoxime andε-caprolactam.

To facilitate the reaction associated with the isocyanategroup-containing organic compound and to obtain higher effects, it ispreferable to use catalysts, for example, metal catalysts such asdibutyltin dilaurate, and amine catalysts such as1,8-diazabicyclo[5.4.0]undecene-7.

The isocyanate group-containing organic compounds may be used singly, ortwo or more may be used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the amount of the isocyanategroup-containing organic compound (the total amount when two or morekinds of the compounds are used) may be 0.001 mass % or above,preferably 0.1 mass % or above, and more preferably 0.3 mass % or above,and may be 10 mass % or less, preferably 5 mass % or less, and morepreferably 3 mass % or less. This amount ensures easy control ofcharacteristics such as output characteristics, load characteristics,low-temperature characteristics, cycle characteristics andhigh-temperature storage characteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the isocyanate group-containing organic compound,namely, the aromatic carboxylate ester of Formula (2):isocyanategroup-containing organic compound ratio is preferably 1:99 to 99:1, morepreferably 5:95 to 95:5, still more preferably 10:90 to 90:10,particularly preferably 20:80 to 80:20, and highly preferably 30:70 to70:30. This ratio ensures that battery characteristics, in particular,initial characteristics will be markedly enhanced. Although themechanism of this effect is not clear, it is probable that the additivesmixed in the above ratio are prevented from side reactions on theelectrodes to the maximum extent.

1-3-6. Silicon-Containing Compounds

The electrolytic solution of the invention may further include asilicon-containing compound. The silicon-containing compounds are notparticularly limited as long as the compounds have at least one siliconatom in the molecule. In the electrolytic solution of the invention, thecombined use of the aromatic carboxylate ester of Formula (2) with thesilicon-containing compound enhances the initial high-rate dischargecapacity and also enhances the capacity after storage.

The silicon-containing compounds are preferably represented by Formula(3-6) below:

(In the formula,

R⁶¹, R⁶² and R⁶³ are independently a hydrogen atom, a halogen atom or ahydrocarbon group having 10 or less carbon atoms, and

X⁶¹ is an organic group containing at least one atom selected from thegroup consisting of oxygen atoms, nitrogen atoms and silicon atoms.)

Examples and preferred examples of the hydrocarbon groups are similar tothose of the hydrocarbon groups in Formula (1). R⁶¹, R⁶² and R⁶³ arepreferably each a hydrogen atom, a fluorine atom, a methyl group, anethyl group, an n-propyl group, an i-propyl group, an n-butyl group, asec-butyl group, an i-butyl group, a tert-butyl group or a phenyl group,and more preferably a methyl group.

X⁶¹ is an organic group containing at least one atom selected from thegroup consisting of oxygen atoms, nitrogen atoms and silicon atoms, andis preferably an organic group containing at least an oxygen atom or asilicon atom. Here, the organic group is a group composed of one or moreatoms selected from the group consisting of carbon atoms, hydrogenatoms, nitrogen atoms, oxygen atoms, silicon atoms, sulfur atoms,phosphorus atoms and halogen atoms. Examples of the organic groupsinclude alkyl groups, alkenyl groups, alkynyl groups, aryl groups,alkoxy groups, CN groups, isocyanate groups, fluoro groups,alkylsulfonate groups and trialkylsilyl groups. The monovalent organicgroup may be partially substituted with a halogen atom. The number ofcarbon atoms in the organic group may be 1 or more, preferably 3 ormore, and more preferably 5 or more, and may be 15 or less, preferably12 or less, and more preferably 8 or less.

Of the organic groups, alkylsulfonate groups, trialkylsilyl groups,borate groups, phosphate groups and phosphite groups are preferable.

Examples of the silicon-containing compounds include the followingcompounds:

borate compounds such as tris(trimethylsilyl) borate,tris(trimethoxysilyl) borate, tris(triethylsilyl) borate,tris(triethoxysilyl) borate, tris(dimethylvinylsilyl) borate andtris(diethylvinylsilyl) borate; phosphate compounds such astris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate,tris(tripropylsilyl) phosphate, tris(triphenylsilyl) phosphate,tris(trimethoxysilyl) phosphate, tris(triethoxysilyl) phosphate,tris(triphenoxysilyl) phosphate, tris(dimethylvinylsilyl) phosphate andtris(diethylvinylsilyl) phosphate;

phosphite compounds such as tris(trimethylsilyl) phosphite,tris(triethylsilyl) phosphite, tris(tripropylsilyl) phosphite,tris(triphenylsilyl) phosphite, tris(trimethoxysilyl) phosphite,tris(triethoxysilyl) phosphite, tris(triphenoxysilyl) phosphite,tris(dimethylvinylsilyl) phosphite and tris(diethylvinylsilyl)phosphite;

sulfonate compounds such as trimethylsilyl methanesulfonate andtrimethylsilyl tetrafluoromethanesulfonate; and

disilane compounds such as hexamethyldisilane, hexaethyldisilane,1,1,2,2-tetramethyldisilane, 1,1,2,2-tetraethyldisilane,1,2-diphenyltetramethyldisilane and 1,1,2,2-tetraphenyldisilane.

Of these, tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate,tris(trimethylsilyl) phosphite, trimethylsilyl methanesulfonate,trimethylsilyl tetrafluoromethanesulfonate, hexamethyldisilane,hexaethyldisilane, 1,2-diphenyltetramethyldisilane and1,1,2,2-tetraphenyldisilane are preferable, and tris(trimethylsilyl)borate, tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphiteand hexamethyldisilane are more preferable.

The silicon-containing compounds may be used singly, or two or more maybe used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the amount of thesilicon-containing compound (the total amount when two or more kinds ofthe compounds are used) may be 0.001 mass % or above, preferably 0.1mass % or above, and more preferably 0.3 mass % or above, and may be 10mass % or less, preferably 5 mass % or less, and more preferably 3 mass% or less. This amount ensures easy control of characteristics such asoutput characteristics, load characteristics, low-temperaturecharacteristics, cycle characteristics and high-temperature storagecharacteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the silicon-containing compound (the total mass when twoor more kinds of the compounds are used), namely, the aromaticcarboxylate ester of Formula (2):silicon-containing compound(s) ratio ispreferably 1:99 to 99:1, more preferably 5:95 to 95:5, still morepreferably 10:90 to 90:10, particularly preferably 20:80 to 80:20, andhighly preferably 30:70 to 70:30. This ratio ensures that batterycharacteristics, in particular, initial characteristics will be markedlyenhanced. Although the mechanism of this effect is not clear, it isprobable that the additives mixed in the above ratio are prevented fromside reactions on the electrodes to the maximum extent.

1-3-7. Aromatic Compounds Other than Those of Formula (2)

The electrolytic solution of the invention may further include anaromatic compound other than those represented by Formula (2).

The aromatic compounds other than those of Formula (2) are notparticularly limited as long as the compounds are organic compoundshaving at least one aromatic ring in the molecule and represented byother than Formula (2). Those aromatic compounds represented by any ofFormulae (3-7-1) and (3-7-2) are preferable.

1-3-7-1. Aromatic Compounds Represented by Formula (3-7-1)

(In the formula, the substituent X⁷¹ is a halogen atom or an organicgroup optionally having a halogen atom or a heteroatom. The organicgroup optionally having a heteroatom is a linear, branched or cyclic,saturated hydrocarbon group having 1 to 12 carbon atoms, a group havinga carboxylate ester structure, a group having a carbonate structure, aphosphorus-containing group, a sulfur-containing group or asilicon-containing group. These substituents may be further substitutedwith a substituent such as a halogen atom, a hydrocarbon group, anaromatic group, a halogen-containing hydrocarbon group or ahalogen-containing aromatic group. n⁷¹ that indicates the number of thesubstituents X⁷¹ is 1 to 6. When the compound has a plurality ofsubstituents, the substituents may be the same as or different from oneanother and may form a ring.)

From the point of view of battery characteristics, linear, branched orcyclic, saturated hydrocarbon groups having 1 to 12 carbon atoms, groupshaving a carboxylate ester structure and groups having a carbonatestructure are preferable. Linear, branched or cyclic, saturatedhydrocarbon groups having 3 to 12 carbon atoms, and groups having acarboxylate ester structure are more preferable.

The number of the substituents X⁷¹ indicated by n⁷¹ is preferably 1 to5, more preferably 1 to 3, still more preferably 1 to 2, andparticularly preferably 1.

X⁷¹ represents a halogen atom, or an organic group optionally having ahalogen atom or a heteroatom.

Examples of the halogen atoms include chlorine and fluorine, withfluorine being preferable.

Examples of the organic groups having no heteroatoms include linear,branched or cyclic, saturated hydrocarbon groups having 3 to 12 carbonatoms. Such linear or branched groups may have a ring structure.Specific examples of the linear, branched or cyclic, saturatedhydrocarbon groups having 1 to 12 carbon atoms include methyl group,ethyl group, propyl group, isopropyl group, butyl group, isobutyl group,tert-butyl group, pentyl group, tert-pentyl group, cyclopentyl group,cyclohexyl group and butylcyclohexyl group. The number of carbon atomsis preferably 3 to 12, more preferably 3 to 10, still more preferably 3to 8, further preferably 3 to 6, and most preferably 3 to 5.

Examples of the heteroatoms present in the organic groups having aheteroatom include oxygen atoms, sulfur atoms, phosphorus atoms andsilicon atoms. Examples of the oxygen-containing organic groups includegroups having a carboxylate ester structure and groups having acarbonate structure. Examples of the sulfur-containing organic groupsinclude groups having a sulfonate ester structure. Examples of thephosphorus-containing organic groups include groups having a phosphateester structure and groups having a phosphonate ester structure.Examples of the silicon-containing organic groups include groups havinga silicon-carbon structure.

Examples of the aromatic compounds represented by Formula (3-7-1)include the following compounds.

Examples of the compounds in which X⁷¹ is a halogen atom or an organicgroup optionally having a halogen atom include:

chlorobenzene, fluorobenzene, difluorobenzene, trifluorobenzene,tetrafluorobenzene, pentafluorobenzene, hexafluorobenzene andbenzotrifluoride. Fluorobenzene and hexafluorobenzene are preferable.Fluorobenzene is more preferable.

Examples of the compounds in which X⁷¹ is a hydrocarbon group having 1to 12 carbon atoms include:

2,2-diphenylpropane, 1,4-diphenylcyclohexane, cyclopentylbenzene,cyclohexylbenzene, cis-1-propyl-4-phenylcyclohexane,trans-1-propyl-4-phenylcyclohexane, cis-1-butyl-4-phenylcyclohexane,trans-1-butyl-4-phenylcyclohexane, propylbenzene, butylbenzene,tert-butylbenzene and tert-amylbenzene. Preferred compounds are2,2-diphenylpropane, 1,4-diphenylcyclohexane, cyclopentylbenzene,cyclohexylbenzene, cis-1-propyl-4-phenylcyclohexane,trans-1-propyl-4-phenylcyclohexane, cis-1-butyl-4-phenylcyclohexane,trans-1-butyl-4-phenylcyclohexane, toluene, ethylbenzene, propylbenzene,butylbenzene, tert-butylbenzene and tert-amylbenzene. More preferredcompounds are 2,2-diphenylpropane, cyclopentylbenzene,cyclohexylbenzene, 1,1-diphenylcyclohexane, tert-butylbenzene andtert-amylbenzene. Cyclohexylbenzene, tert-butylbenzene andtert-amylbenzene are still more preferable.

Examples of the compounds in which X⁷¹ is a group having a carboxylateester structure include:

phenyl acetate, benzyl acetate, 2-phenylethyl acetate, 3-phenylpropylacetate, 4-phenylbutyl acetate, phenyl propionate, benzyl propionate,2-phenylethyl propionate, 3-phenylpropyl propionate, 4-phenylbutylpropionate, phenyl butyrate, benzyl butyrate, 2-phenylethyl butyrate,3-phenylpropyl butyrate, 4-phenylbutyl butyrate, phenethyl phenylacetateand 2,2-bis(4-acetoxyphenyl)propane. Preferred compounds are2-phenylethyl acetate, 3-phenylpropyl acetate, 2-phenylethyl propionate,3-phenylpropyl propionate and 2,2-bis(4-acetoxyphenyl)propane.2-Phenylethyl acetate and 3-phenylpropyl acetate are more preferable.

Examples of the compounds in which X⁷¹ is a group having a carbonatestructure include:

2,2-bis(4-methoxycarbonyloxyphenyl)propane,1,1-bis(4-methoxycarbonyloxyphenyl)cyclohexane, diphenyl carbonate,methyl phenyl carbonate, ethyl phenyl carbonate, 2-tert-butylphenylmethyl carbonate, 2-tert-butylphenyl ethyl carbonate,bis(2-tert-butylphenyl) carbonate, 4-tert-butylphenyl methyl carbonate,4-tert-butylphenyl ethyl carbonate, bis(4-tert-butylphenyl) carbonate,benzyl methyl carbonate, benzyl ethyl carbonate and dibenzyl carbonate.Preferred compounds are 2,2-bis(4-methoxycarbonyloxyphenyl)propane,1,1-bis(4-methoxycarbonyloxyphenyl)cyclohexane, diphenyl carbonate andmethyl phenyl carbonate. Diphenyl carbonate and methyl phenyl carbonateare more preferable. Methyl phenyl carbonate is still more preferable.

Examples of the compounds in which X⁷¹ is a group having a sulfonateester structure include:

methyl phenylsulfonate, ethyl phenylsulfonate, diphenyl sulfonate,phenyl methylsulfonate, 2-tert-butylphenyl methylsulfonate,4-tert-butylphenyl methylsulfonate and cyclohexylphenyl methylsulfonate.Preferred compounds are methyl phenylsulfonate, diphenyl sulfonate,2-tert-butylphenyl methylsulfonate, 4-tert-butylphenyl methylsulfonateand cyclohexylphenyl methylsulfonate. More preferred compounds aremethyl phenylsulfonate, 2-tert-butylphenyl methylsulfonate,4-tert-butylphenyl methylsulfonate and cyclohexylphenyl methylsulfonate.

Examples of the compounds in which X⁷¹ is a group having asilicon-carbon structure include:

trimethylphenylsilane, diphenylsilane and diphenyltetramethyldisilane.Trimethylphenylsilane is preferable.

Examples of the compounds in which X⁷¹ is a group having a phosphateester structure include:

triphenyl phosphate, tris(2-tert-butylphenyl) phosphate,tris(3-tert-butylphenyl) phosphate, tris(4-tert-butylphenyl) phosphate,tris(2-tert-amylphenyl) phosphate, tris(3-tert-amylphenyl) phosphate,tris(4-tert-amylphenyl) phosphate, tris(2-cyclohexylphenyl) phosphate,tris(3-cyclohexylphenyl) phosphate, tris(4-cyclohexylphenyl) phosphateand diethyl(4-methylbenzyl) phosphonate. Preferred compounds aretriphenyl phosphate, tris(2-tert-butylphenyl) phosphate,tris(3-tert-butylphenyl) phosphate, tris(4-tert-butylphenyl) phosphate,tris(2-tert-amylphenyl) phosphate, tris(3-tert-amylphenyl) phosphate,tris(4-tert-amylphenyl) phosphate, tris(2-cyclohexylphenyl) phosphate,tris(3-cyclohexylphenyl) phosphate and tris(4-cyclohexylphenyl)phosphate. More preferred compounds are tris(2-tert-butylphenyl)phosphate, tris(4-tert-butylphenyl) phosphate, tris(2-cyclohexylphenyl)phosphate and tris(4-cyclohexylphenyl) phosphate.

Examples of the compounds in which X⁷¹ is a group having a phosphonateester structure include:

dimethyl phenylphosphonate, diethyl phenylphosphonate, methyl phenylphenylphosphonate, ethyl phenyl phenylphosphonate, diphenylphenylphosphonate, dimethyl-(4-fluorophenyl)-phosphonate, dimethylbenzylphosphonate, diethyl benzylphosphonate, methyl phenylbenzylphosphonate, ethyl phenyl benzylphosphonate, diphenylbenzylphosphonate, dimethyl-(4-fluorobenzyl)phosphonate anddiethyl-(4-fluorobenzyl)phosphonate. Preferred compounds are dimethylphenylphosphonate, diethyl phenylphosphonate,dimethyl-(4-fluorophenyl)-phosphonate, dimethyl benzylphosphonate,diethyl benzylphosphonate, dimethyl-(4-fluorobenzyl)phosphonate anddiethyl-(4-fluorobenzyl)phosphonate. More preferred compounds aredimethyl phenylphosphonate, diethyl phenylphosphonate, dimethylbenzylphosphonate, diethyl benzylphosphonate,dimethyl-(4-fluorobenzyl)phosphonate anddiethyl-(4-fluorobenzyl)phosphonate.

Examples of the aromatic compounds represented by Formula (3-7-1)further include fluorides of the above aromatic compounds. Specificexamples include:

partial fluorides of the compounds having a hydrocarbon group such as2-fluorotoluene, 3-fluorotoluene, 4-fluorotoluene,trifluoromethylbenzene, o-cyclohexylfluorobenzene andp-cyclohexylfluorobenzene; partial fluorides of the compounds having acarboxylate ester structure such as 2-fluorophenyl acetate and4-fluorophenyl acetate; and partial fluorides of the compounds having anether structure such as trifluoromethoxybenzene, 2-fluoroanisole,3-fluoroanisole, 4-fluoroanisole, 2,4-difluoroanisole,2,5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole and4-trifluoromethoxyanisole. Preferred compounds are partial fluorides ofthe compounds having a hydrocarbon group such as trifluoromethylbenzene,2-fluorotoluene, 3-fluorotoluene, 4-fluorotoluene,o-cyclohexylfluorobenzene and p-cyclohexylfluorobenzene; partialfluorides of the compounds having a carboxylate ester structure such as2-fluorophenyl acetate and 4-fluorophenyl acetate; and partial fluoridesof the compounds having an ether structure such astrifluoromethoxybenzene 2-fluoroanisole, 4-fluoroanisole,2,4-difluoroanisole and 4-trifluoromethoxyanisole. More preferredcompounds are partial fluorides of the compounds having a hydrocarbongroup such as 2-fluorotoluene, 3-fluorotoluene and 4-fluorotoluene;partial fluorides of the compounds having a carboxylate ester structuresuch as 2-fluorophenyl acetate and 4-fluorophenyl acetate; and partialfluorides of the compounds having an ether structure such astrifluoromethoxybenzene, 2-fluoroanisole, 4-fluoroanisole,2,4-difluoroanisole and 4-trifluoromethoxyanisole.

The aromatic compound represented by Formula (3-7-1) may be used singly,or two or more may be used in combination in an appropriate ratio. In100 mass % of the electrolytic solution, the amount of the aromaticcompound represented by Formula (3-7-1) (the total amount when two ormore kinds of the compounds are used) may be 0.001 mass % or above,preferably 0.01 mass % or above, more preferably 0.1 mass % or above,and still more preferably 0.4 mass % or above, and may be 10 mass % orless, preferably 8 mass % or less, more preferably 5 mass % or less,still more preferably 4 mass % or less, and particularly preferably 4mass % or less. This amount ensures that the advantageous effects of theinvention will be obtained prominently and the increase in batteryresistance will be prevented.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the aromatic compound represented by Formula (3-7-1)(the total mass when two or more kinds of the compounds are used) ispreferably 1:99 to 99:1, more preferably 10:90 to 90:10, andparticularly preferably 20:80 to 80:20. This ratio ensures thatovercharge characteristics may be enhanced without a decrease in batterycharacteristics.

1-3-7-2. Aromatic Compounds Represented by Formula (3-7-2)

(In the formula, R¹¹ to R¹⁵ are independently hydrogen, a halogen, or anunsubstituted or halogenated hydrocarbon group having 1 to 20 carbonatoms; R¹⁶ and R¹⁷ are independently a hydrocarbon group having 1 to 12carbon atoms; and at least two of R¹¹ to R¹⁷ may be bonded together toform a ring. Formula (3-7-2) satisfies at least one of the requirements(A) and (B):

(A) At least one of R¹¹ to R¹⁵ is a halogen, or an unsubstituted orhalogenated hydrocarbon group having 1 to 20 carbon atoms.

(B) The total number of carbon atoms in R¹¹ to R¹⁷ is 3 to 20.) When atleast two of R¹¹ to R¹⁷ are bonded together to form a ring, it ispreferable that the ring be formed by two of R¹¹ to R¹⁷.

R¹⁶ and R¹⁷ are independently a hydrocarbon group having 1 to 12 carbonatoms (for example, an alkyl group or an aryl group). R¹⁶ and R¹⁷ may bebonded together to form a ring (for example, a cyclic hydrocarbongroup). To obtain enhancements in initial efficiency, solubility andstorage characteristics, R¹⁶ and R¹⁷ are preferably hydrocarbon groupshaving 1 to 12 carbon atoms or are bonded to each other to form a cyclichydrocarbon group; R¹⁶ and R¹⁷ are more preferably each a methyl group,an ethyl group, a propyl group, a butyl group or a tert-butyl group orare bonded together to form a 5- to 8-membered cyclic hydrocarbon group;R¹⁶ and R¹⁷ are still more preferably each a methyl group or an ethylgroup or are bonded together to form a cyclohexyl group or a cyclopentylgroup; and R¹⁶ and R¹⁷ are most preferably each a methyl group or anethyl group or are bonded together to form a cyclohexyl group.

R¹¹ to R¹⁵ are independently hydrogen, a halogen, or an unsubstituted orhalogenated hydrocarbon group having 1 to 20 carbon atoms (for example,an alkyl group, an aryl group or an aralkyl group). Two of thesesubstituents may be bonded together to form a ring (for example, acyclic hydrocarbon group). To obtain enhancements in initial efficiency,solubility and storage characteristics, these substituents arepreferably each hydrogen, fluorine, or an unsubstituted or halogenatedhydrocarbon group having 1 to 12 carbon atoms, more preferably eachhydrogen, fluorine, or an unsubstituted or fluorinated hydrocarbon grouphaving 1 to 10 carbon atoms, still more preferably each hydrogen,fluorine, a tert-butyl group, a tert-pentyl group, a tert-hexyl group, atert-heptyl group, a methyl group, an ethyl group, a propyl group, abutyl group, a trifluoromethyl group, a nonafluoro-tert-butyl group, a1-methyl-1-phenyl-ethyl group or a 1-ethyl-1-phenyl-propyl group,particularly preferably each hydrogen, fluorine, a tert-butyl group or a1-methyl-1-phenyl-ethyl group, and most preferably each hydrogen, atert-butyl group or a 1-methyl-1-phenyl-ethyl group.

One of R¹¹ to R¹⁵, and R¹⁶ may be bonded together to form a ring (forexample, a cyclic hydrocarbon group). Preferably, R¹¹ and R¹⁶ are bondedtogether to form a ring (for example, a cyclic hydrocarbon group). Inthis case, R¹⁷ is preferably an alkyl group. Examples of the compoundsin which R¹⁷ is a methyl group, and R¹¹ and R¹⁶ form a ring togetherinclude 1-phenyl-1,3,3-trimethylindane and2,3-dihydro-1,3-dimethyl-1-(2-methyl-2-phenylpropyl)-3-phenyl-1H-indane.

Formula (3-7-2) satisfies at least one of the requirements (A) and (B):

(A) At least one of R¹¹ to R¹⁵ is a halogen, or an unsubstituted orhalogenated hydrocarbon group having 1 to 20 carbon atoms.

(B) The total number of carbon atoms in R¹¹ to R¹⁷ is 3 to 20.

From the point of view of the suppression of oxidation on the positiveelectrode at normal battery operation voltages, it is preferable thatFormula (3-7-2) satisfy the requirement (A). From the point of view ofthe solubility in the electrolytic solution, it is preferable that theformula satisfy the requirement (B). Formula (3-7-2) may satisfy boththe requirements (A) and (B).

As long as the requirement (A) is satisfied, specifically, as long as atleast one of R¹¹ to R¹⁵ is a halogen, or an unsubstituted or halogenatedhydrocarbon group having 1 to 20 carbon atoms, the other substituentsmay be hydrogen atoms or may form a ring. From the point of view of thesolubility in the electrolytic solution, the number of carbon atoms inthe unsubstituted or halogenated hydrocarbon group is preferably 1 to10, more preferably 1 to 5, still more preferably 1 to 3, furtherpreferably 1 or 2, and most preferably 1.

As long as the requirement (B) is satisfied, specifically, as long asthe total number of carbon atoms in R¹¹ to R¹⁷ is 3 to 20, at least twoof R¹¹ to R¹⁷ may be bonded together to form a ring. When at least twoof R¹¹ to R¹⁷ are bonded to each other to form a ring, the calculationof the total number of carbon atoms neglects the carbon atoms in thering that do not correspond to R¹¹ to R¹⁷ (the carbon atoms in thebenzene ring to which R¹¹ to R¹⁵ are bonded, and the benzyl carbon atomto which R¹⁶ and R¹⁷ are bonded). From the point of view of thesolubility in the electrolytic solution, the total number of carbonatoms is preferably 3 to 14, and more preferably 3 to 10. Some of thecompounds satisfying the requirement (B) are1-phenyl-1,3,3-trimethylindane and2,3-dihydro-1,3-dimethyl-1-(2-methyl-2-phenylpropyl)-3-phenyl-1H-indanementioned above as examples of the compounds in which R¹⁷ is a methylgroup, and R¹¹ and R¹⁶ form a ring together.

Examples of the aromatic compounds represented by Formula (3-7-2)include the following:

those compounds in which R¹⁶ and R¹⁷ are independently a hydrocarbongroup having 1 to 20 carbon atoms (with the proviso that the totalnumber of carbon atoms in R¹⁶ and R¹⁷ is 3 to 20), and R¹¹ to R¹⁵ arehydrogen (satisfying the requirement (B));

2,2-diphenylbutane, 3,3-diphenylpentane, 3,3-diphenylhexane,4,4-diphenylheptane, 5,5-diphenyloctane, 6,6-diphenylnonane,1,1-diphenyl-1,1-di-tert-butyl-methane;

those compounds in which R¹⁶ and R¹⁷ are bonded together to form a ring,and R¹¹ to R¹⁵ are hydrogen (satisfying the requirement (B));

1,1-diphenylcyclohexane, 1,1-diphenylcyclopentane and1,1-diphenyl-4-methylcyclohexane.

Examples further include the following compounds (some of the compoundsillustrated below are the same as those mentioned above):

Examples further include those compounds in which at least one of R¹¹ toR¹⁵ is a halogen, or an unsubstituted or halogenated hydrocarbon grouphaving 1 to 20 carbon atoms (satisfying the requirement (A));

1,3-bis(1-methyl-1-phenylethyl)-benzene and1,4-bis(1-methyl-1-phenylethyl)-benzene.

Examples further include the following compounds (some of the compoundsillustrated below are the same as those mentioned above):

Examples further include those compounds in which R¹⁷ is a hydrocarbongroup having 1 to 20 carbon atoms (for example, an alkyl group having 1to 20 carbon atoms, preferably a methyl group), and R¹¹ and R¹⁶ arebonded together to form a ring (satisfying the requirement (B)); and

1-phenyl-1,3,3-trimethylindane.

Examples further include the following compounds (some of the compoundsillustrated below are the same as those mentioned above):

In particular, 2,2-diphenylbutane, 3,3-diphenylpentane,1,1-diphenyl-1,1-di-tert-butyl-methane, 1,1-diphenylcyclohexane,1,1-diphenylcyclopentane, 1,1-diphenyl-4-methylcyclohexane,1,3-bis(1-methyl-1-phenylethyl)-benzene,1,4-bis(1-methyl-1-phenylethyl)-benzene and1-phenyl-1,3,3-trimethylindane are preferable from the point of view ofthe initial reducibility on the negative electrode.

Preferred examples further include the following compounds (some of thecompounds illustrated below are the same as the above preferredcompounds):

More preferred compounds are 2,2-diphenylbutane,1,1-diphenylcyclohexane, 1,1-diphenyl-4-methylcyclohexane,1,3-bis(1-methyl-1-phenylethyl)-benzene,1,4-bis(1-methyl-1-phenylethyl)-benzene and1-phenyl-1,3,3-trimethylindane.

More preferred examples further include the following compounds (some ofthe compounds illustrated below are the same as the above more preferredcompounds):

Still more preferred compounds are 1,1-diphenylcyclohexane,1,1-diphenyl-4-methylcyclohexane,1,3-bis(l-methyl-1-phenylethyl)-benzene,1,4-bis(1-methyl-1-phenylethyl)-benzene and1-phenyl-1,3,3-trimethylindane.

Still more preferred examples further include the following compounds(some of the compounds illustrated below are the same as the above stillmore preferred compounds):

Particularly preferred compounds are 1,1-diphenylcyclohexane,1,3-bis(1-methyl-1-phenylethyl)-benzene,1,4-bis(1-methyl-1-phenylethyl)-benzene and1-phenyl-1,3,3-trimethylindane represented by the following structuralformulae:

The most preferred compound is l-phenyl-1,3,3-trimethylindane compoundrepresented by the following structural formula:

The aromatic compounds represented by Formula (3-7-2) may be usedsingly, or two or more may be used in combination. In the whole amountof the nonaqueous electrolytic solution (100 mass %), the amount of thearomatic compound represented by Formula (3-7-2) (the total amount whentwo or more kinds of the compounds are used) may be 0.001 mass % orabove, preferably 0.01 mass % or above, more preferably 0.05 mass % orabove, and still more preferably 0.1 mass % or above, and may be 10 mass% or less, preferably 8 mass % or less, more preferably 5 mass % orless, still more preferably 3 mass % or less, and particularlypreferably 2.5 mass % or less. This amount ensures that the advantageouseffects of the invention will be obtained prominently and the increasein battery resistance can be prevented.

1-3-8. Carboxylate Esters Represented by Formula (3)

(In Formula (3), R⁵ is a hydrocarbon group having 1 to 4 carbon atoms,and R⁶ is an ethyl group, an n-propyl group or an n-butyl group.)

The hydrocarbon group with 1 to 4 carbon atoms represented by R⁵ is notparticularly limited. The number of carbon atoms is usually 1 or more,and preferably 2 or more, and is usually 4 or less, and preferably 3 orless. Specific examples of the hydrocarbon groups include alkyl groups,alkenyl groups and alkynyl groups. Of these, preferred groups are alkylgroups having 1 to 4 carbon atoms such as methyl group, ethyl group,n-propyl group, i-propyl group, n-butyl group, sec-butyl group, i-butylgroup and tert-butyl group; alkenyl groups having 2 to 3 carbon atomssuch as vinyl group, 1-propenyl group, 2-propenyl group, isopropenylgroup, 1-butenyl group, 2-butenyl group and 3-butenyl group; and alkynylgroups having 2 to 4 carbon atoms such as ethynyl group, 1-propynylgroup, 2-propynyl group, 1-butynyl group, 2-butynyl group and 3-butynylgroup. Alkyl groups having 1 to 4 carbon atoms such as methyl group,ethyl group, n-propyl group, i-propyl group, n-butyl group, sec-butylgroup, i-butyl group and tert-butyl group are more preferable. Methylgroup, ethyl group, n-propyl group and n-butyl group are still morepreferable, and ethyl group and n-propyl group are particularlypreferable.

R⁶ is an ethyl group, an n-propyl group or an n-butyl group, preferablyan ethyl group or an n-propyl group, and more preferably an ethyl group.

Examples of the carboxylate esters represented by Formula (3) includeethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate,n-propyl propionate, n-butyl propionate, ethyl butyrate, n-propylbutyrate, n-butyl butyrate, ethyl isobutyrate, n-propyl isobutyrate,n-butyl isobutyrate, ethyl valerate, n-propyl valerate, n-butylvalerate, ethyl hydroangelate, n-propyl hydroangelate, n-butylhydroangelate, ethyl isovalerate, n-propyl isovalerate, n-butylisovalerate, ethyl pivalate, n-propyl pivalate, n-butyl pivalate, ethylacrylate, n-propyl acrylate, n-butyl acrylate, ethyl methacrylate,n-propyl methacrylate, n-butyl methacrylate, ethyl crotonate, n-propylcrotonate, n-butyl crotonate, ethyl 3-butenoate, n-propyl 3-butenoate,n-butyl 3-butenoate, ethyl 4-pentenoate, n-propyl 4-pentenoate, n-butyl4-pentenoate, ethyl 3-pentenoate, n-propyl 3-pentenoate, n-butyl3-pentenoate, ethyl 2-pentenoate, n-propyl 2-pentenoate, n-butyl2-pentenoate, ethyl 2-propiolate, n-propyl 2-propiolate, n-butyl2-propiolate, ethyl 3-butynoate, n-propyl 3-butynoate, n-butyl3-butynoate, ethyl 2-butynoate, n-propyl 2-butynoate, n-butyl2-butynoate, ethyl 4-pentynoate, n-propyl 4-pentynoate, n-butyl4-pentynoate, ethyl 3-pentynoate, n-propyl 3-pentynoate, n-butyl3-pentynoate, ethyl 2-pentynoate, n-propyl 2-pentynoate and n-butyl2-pentynoate.

Of these, ethyl acetate, n-propyl acetate, n-butyl acetate, ethylpropionate, n-propyl propionate, n-butyl propionate, ethyl butyrate,n-propyl butyrate, n-butyl butyrate, ethyl isobutyrate, n-propylisobutyrate, n-butyl isobutyrate, ethyl valerate, n-propyl valerate,n-butyl valerate, ethyl hydroangelate, n-propyl hydroangelate, n-butylhydroangelate, ethyl isovalerate, n-propyl isovalerate, n-butylisovalerate, ethyl pivalate, n-propyl pivalate and n-butyl pivalate arepreferable in order to obtain enhancements in initial characteristics.Ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate,n-propyl propionate, n-butyl propionate, ethyl butyrate, n-propylbutyrate, n-butyl butyrate, ethyl valerate, n-propyl valerate, n-butylvalerate, ethyl isovalerate, n-propyl isovalerate and n-butylisovalerate are preferable. Ethyl propionate, n-propyl propionate,n-butyl propionate, ethyl butyrate, n-propyl butyrate, n-butyl butyrate,ethyl valerate, n-propyl valerate and n-butyl valerate are morepreferable. Ethyl propionate, n-propyl propionate, n-butyl propionate,ethyl butyrate, n-propyl butyrate and n-butyl butyrate are still morepreferable.

The carboxylate esters represented by Formula (3) may be used singly, ortwo or more may be used in combination in an appropriate ratio.

In 100 mass % of the electrolytic solution, the amount of thecarboxylate ester represented by Formula (3) (the total amount when twoor more kinds of the esters are used) is preferably 0.1 mass % or above,more preferably 0.3 mass % or above, and still more preferably 0.4 mass% or above, and is preferably 10 mass % or less, more preferably 5 mass% or less, still more preferably 3 mass % or less, further preferably 2mass % or less, and particularly preferably 1 mass % or less. When thecarboxylate ester represented by Formula (3) is used as a nonaqueoussolvent, the ratio thereof to the total nonaqueous solvent taken as 100vol % is preferably 1 vol % or above, more preferably 5 vol % or above,still more preferably 10 vol % or above, and further preferably 20 vol %or above, and may be 50 vol % or less, more preferably 45 vol % or less,and still more preferably 40 vol % or less.

From the point of view of the formation of a composite interfaceprotective film on the negative electrode, the mass ratio of thearomatic carboxylate ester represented by Formula (2) to the carboxylateester represented by Formula (3) is preferably 1:99 to 99:1, morepreferably 5:95 to 95:5, still more preferably 10:90 to 90:10,particularly preferably 20:80 to 80:20, and highly preferably 30:70 to70:30. This ratio ensures that side reactions of the additives on thepositive and negative electrodes can be prevented efficiently andbattery characteristics will be enhanced. In particular, the above ratiois useful in order to suppress a decrease in initial characteristics andto enhance safety during overcharging.

1-3-9. Cyclic Compounds Having Plurality of Ether Bonds

The cyclic compounds having a plurality of ether bonds are notparticularly limited as long as the compounds are cyclic and have aplurality of ether bonds in the molecule. Compounds represented byFormula (3-9) are preferable. The cyclic compounds having a plurality ofether bonds contribute to the improvement in high-temperature storagecharacteristics of batteries. In the electrolytic solution of theinvention, the combined use thereof with the aromatic carboxylate esterof Formula (2) also provides good initial characteristics.

(In the formula,

A¹⁵ to A²⁰ are independently a hydrogen atom, a fluorine atom or anoptionally substituted hydrocarbon group having 1 to 5 carbon atoms, andn¹⁰¹ is an integer of 1 to 4. When n¹⁰¹ is an integer of 2 or greater,the pluralities of A¹⁷ and A¹⁸ may be the same as or different from oneanother.)

Any two selected from A¹⁵ to A²⁰ may be bonded together to form a ring.In this case, it is preferable that A¹⁷ and A¹⁸ form a ring structure.The total number of carbon atoms in A¹⁵ to A²⁰ is preferably 0 to 8,more preferably 0 to 4, still more preferably 0 to 2, and particularlypreferably 0 to 1.

Examples of the substituents include halogen atoms, optionallyhalogenated alkyl, alkenyl, alkynyl, aryl and alkoxy groups, cyanogroups, isocyanate groups, ether groups, carbonate groups, carbonylgroups, carboxyl groups, alkoxycarbonyl groups, acyloxy groups, sulfonylgroups, phosphanetriyl groups and phosphoryl groups. Of these, halogenatoms, alkoxy groups, optionally halogenated alkyl, alkenyl and alkynylgroups, isocyanate groups, cyano groups, ether groups, carbonyl groups,alkoxycarbonyl groups and acyloxy groups are preferable. Unhalogenatedalkyl groups, cyano groups and ether groups are more preferable.

In Formula (3-9), n¹⁰¹ is preferably an integer of 1 to 3, and morepreferably an integer of 1 to 2. Still more preferably, n¹⁰¹ is 2.

Examples of the hydrocarbon groups with 1 to 5 carbon atoms representedby A¹⁵ to A²⁰ include monovalent hydrocarbon groups such as alkylgroups, alkenyl groups, alkynyl groups and aryl groups; and

divalent hydrocarbon groups such as alkylene groups, alkenylene groups,alkynylene groups and arylene groups. Of these, alkyl groups andalkylene groups are preferable, and alkyl groups are more preferable.Specific examples include:

alkyl groups having 1 to 5 carbon atoms such as methyl group, ethylgroup, n-propyl group, i-propyl group, n-butyl group, sec-butyl group,i-butyl group, tert-butyl group, n-pentyl group, isopentyl group,sec-pentyl group, neopentyl group, 1-methylbutyl group, 2-methylbutylgroup, 1,1-dimethylpropyl group and 1,2-dimethylpropyl group;

alkenyl groups having 2 to 5 carbon atoms such as vinyl group,I-propenyl group, 2-propenyl group, isopropenyl group, 1-butenyl group,2-butenyl group, 3-butenyl group, 1-pentenyl group, 2-pentenyl group,3-pentenyl group and 4-pentenyl group;

alkynyl groups having 2 to 5 carbon atoms such as ethynyl group,1-propynyl group, 2-propynyl group, 1-butynyl group, 2-butynyl group,3-butynyl group, 1-pentynyl group, 2-pentynyl group, 3-pentynyl groupand 4-pentynyl group;

alkylene groups having 1 to 5 carbon atoms such as methylene group,ethylene group, trimethylene group, tetramethylene group andpentamethylene group;

alkenylene groups having 2 to 5 carbon atoms such as vinylene group,1-propenylene group, 2-propenylene group, 1-butenylene group,2-butenylene group, 1-pentenylene group and 2-pentenylene group; and

alkynylene groups having 2 to 5 carbon atoms such as ethynylene group,propynylene group, 1-butynylene group, 2-butynylene group, 1-pentylenegroup and 2-pentylene group. Of these, preferred groups are alkylenegroups having 1 to 5 carbon atoms such as methylene group, ethylenegroup, trimethylene group, tetramethylene group and pentamethylenegroup. Alkylene groups having 2 to 5 carbon atoms such as ethylenegroup, trimethylene group, tetramethylene group and pentamethylene groupare more preferable. Alkylene groups having 3 to 5 carbon atoms such astrimethylene group, tetramethylene group and pentamethylene group arestill more preferable.

A¹⁵ to A²⁰ represent hydrogen atoms, fluorine atoms or hydrocarbongroups having 1 to 5 carbon atoms, specifically, hydrogen atoms,fluorine atoms or combinations of the aforementioned substituents andthe above hydrocarbon groups having 1 to 5 carbon atoms. They preferablyrepresent hydrogen atoms, unsubstituted hydrocarbon groups having 1 to 5carbon atoms or etherified alkylene groups in which the carbon chains ofthe alkylene groups are partially substituted by ether groups, and morepreferably represent hydrogen atoms.

Examples of the cyclic compounds having a plurality of ether bondsinclude the following compounds:

In particular, preferred compounds are:

More preferred compounds are:

The cyclic compounds having a plurality of ether bonds may be usedsingly, or two or more may be used in combination in an appropriateratio. In 100 mass % of the electrolytic solution, the amount of thecyclic compound having a plurality of ether bonds (the total amount whentwo or more kinds of the compounds are used) may be 0.001 mass % orabove, preferably 0.01 mass % or above, more preferably 0.1 mass % orabove, and particularly preferably 0.3 mass % or above, and may be 10mass % or less, preferably 5 mass % or less, more preferably 3 mass % orless, and still more preferably 2 mass % or less. This amount ensureseasy control of characteristics such as output characteristics, loadcharacteristics, low-temperature characteristics, cycle characteristicsand high-temperature storage characteristics.

1-4. Electrolytes

The electrolytes are not particularly limited, and known electrolytesmay be used appropriately. In the case of lithium secondary batteries,lithium salts are usually used. Specific examples include inorganiclithium salts such as LiPF₆, LiBF₄, LiClO₄, LiAlF₄, LiSbF₆, LiTaF₆ andLiWF₇; lithium tungsten oxides such as LiWOF₅; lithium carboxylate saltssuch as HCO₂Li, CH₃CO₂Li, CH₂FCO₂Li, CHF₂CO₂Li, CF₃CO₂Li, CF₃CH₂CO₂Li,CF₃CF₂CO₂Li, CF₃CF₂CF₂CO₂Li and CF₃CF₂CF₂CF₂CO₂Li; lithium sulfonatesalts such as FSO₃Li, CH₃SO₃Li, CH₂FSO₃Li, CHF₂SO₃Li, CF₃SO₃Li,CF₃CF₂SO₃Li, CF₃CF₂CF₂SO₃Li and CF₃CF₂CF₂CF₂SO₃Li; lithium imide saltssuch as LiN(FCO)₂, LiN(FCO)(FSO₂), LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂),LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide and LiN(CF₃SO₂)(C₄F₉SO₂); lithiummethide salts such as LiC(FSO₂)₃, LiC(CF₃SO₂)₃ and LiC(C₂F₅SO₂)₃;lithium (malonato)borate salts such as lithium bis(malonato)borate andlithium difluoro(malonato)borate; lithium (malonato)phosphate salts suchas lithium tris(malonato)phosphate, lithiumdifluorobis(malonato)phosphate and lithiumtetrafluoro(malonato)phosphate; fluorine-containing organolithium saltssuch as LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(CF₃SO₂)₂, LiPF₄(C₂F₅SO₂)₂,LiBF₃CF₃, LiBF₃C₂F₅, LiBF₃C₃F₇, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂,LiBF₂(CF₃SO₂)₂ and LiBF₂(C₂F₅SO₂)₂; lithium oxalatoborate salts such aslithium difluorooxalatoborate and lithium bis(oxalato)borate; and

lithium oxalatophosphate salts such as lithiumtetrafluorooxalatophosphate, lithium difluorobis(oxalato)phosphate andlithium tris(oxalato)phosphate.

In particular, for example, LiPF₆, LiSbF₆, LiTaF₆, FSO₃Li, CF₃SO₃Li,LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithiumcyclic 1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide, LiC(FSO₂)₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃, lithiumdifluorooxalatoborate, lithium bis(oxalato)borate and lithiumdifluorobis(oxalato)phosphate are particularly preferable because oftheir effects of enhancing characteristics such as outputcharacteristics, high-rate charge/discharge characteristics,high-temperature storage characteristics and cycle characteristics.

The concentration of these electrolytes in the nonaqueous electrolyticsolution is not particularly limited as long as the advantageous effectsof the invention are not impaired. In order to ensure good electricconductivity of the electrolytic solution and to ensure good batteryperformance, the total molar concentration of lithium in the nonaqueouselectrolytic solution is preferably 0.3 mol/L or above, more preferably0.4 mol/L or above, and still more preferably 0.5 mol/L or above, and ispreferably 3 mol/L or less, more preferably 2.5 mol/L or less, and stillmore preferably 2.0 mol/L or less. With this concentration, theelectrolytic solution contains an appropriate amount of lithium ions ascharged particles and also exhibits an appropriate viscosity. Thus, goodelectric conductivity is easily ensured.

When two or more kinds of the electrolytes are used in combination, itis preferable that at least one be a salt selected from the groupconsisting of monofluorophosphate salts, difluorophosphate salts, boratesalts, oxalate salts and fluorosulfonate salts, and it is morepreferable that at least one be a salt selected from the groupconsisting of monofluorophosphate salts, difluorophosphate salts,oxalate salts and fluorosulfonate salts. Of these, lithium salts arepreferred. The amount of the salt selected from the group consisting ofmonofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts may be 0.01 mass % or above, andpreferably 0.1 mass % or above, and may be 20 mass % or less, andpreferably 10 mass % or less.

It is preferable that the electrolytes include one or more saltsselected from the group consisting of monofluorophosphate salts,difluorophosphate salts, borate salts, oxalate salts and fluorosulfonatesalts, and one or more additional salts. Examples of the additionalsalts include the lithium salts described above. In particular,preferred salts are LiPF₆, LiN(FSO₂)CF₃SO₂), LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, lithium cyclic 1,2-perfluoroethanedisulfonylimide,lithium cyclic 1,3-perfluoropropanedisulfonylimide, LiC(FSO₂)₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃(CF₃)₃ andLiPF₃(C₂F₅)₃, with LiPF₆ being more preferable. In order to ensure anappropriate balance between the conductivity and the viscosity of theelectrolytic solution, the amount of the additional salt may be 0.01mass % or above, and preferably 0.1 mass % or above, and may be 20 mass% or less, preferably 15 mass % or less, and more preferably 10 mass %or less.

To ensure good battery performance, the total amount of theelectrolyte(s) in the nonaqueous electrolytic solution is preferably 0.3mol/L or above, more preferably 0.4 mol/L or above, and still morepreferably 0.5 mol/L or above, and is preferably 3 mol/L or less, morepreferably 2.5 mol/L or less, still more preferably 2.0 mol/L or less,and particularly preferably 1.5 mol/L or less.

1-4-1. Monofluorophosphate Salts and Difluorophosphate Salts

The monofluorophosphate salts and the difluorophosphate salts are notparticularly limited as long as the salts have at least onemonofluorophosphate or difluorophosphate structure in the molecule. Inthe electrolytic solution of the invention, the combined use of thearomatic carboxylate ester of Formula (2) and one or more of themonofluorophosphate salts and the difluorophosphate salts results in amarked reduction in volume change after initial charging and dischargingof batteries, and a further enhancement in overcharge safety. Further,the combined use makes it possible to reduce the initial irreversiblecapacity of batteries and to enhance discharge storage characteristics.At the same time, the batteries exhibit excellent high-temperature cyclecharacteristics.

The counter cations in the monofluorophosphate salts and thedifluorophosphate salts are not particularly limited. Examples thereofinclude lithium, sodium, potassium, magnesium, calcium and ammoniumrepresented by NR¹²¹R¹²²R¹²³R¹²⁴ (wherein R¹²¹ to R¹²⁴ are independentlya hydrogen atom or an organic group having 1 to 12 carbon atoms). Theorganic groups with 1 to 12 carbon atoms represented by R¹²¹ to R¹²⁴ inthe ammonium are not particularly limited. Examples thereof includeoptionally halogenated alkyl groups, optionally halogenated or alkylatedcycloalkyl groups, optionally halogenated or alkylated aryl groups, andoptionally substituted, nitrogen-containing heterocyclic groups. Inparticular, it is preferable that R¹²¹ to R¹²⁴ be independently ahydrogen atom, an alkyl group, a cycloalkyl group, a nitrogen-containingheterocyclic group or the like. Preferred counter cations are lithium,sodium and potassium. Lithium is particularly preferable.

Examples of the monofluorophosphate salts and the difluorophosphatesalts include lithium monofluorophosphate, sodium monofluorophosphate,potassium monofluorophosphate, lithium difluorophosphate, sodiumdifluorophosphate and potassium difluorophosphate. Lithiummonofluorophosphate and lithium difluorophosphate are preferable, andlithium difluorophosphate is more preferable.

The monofluorophosphate salts and the difluorophosphate salts may beused singly, or two or more may be used in combination in an appropriateratio.

The amount of one or more salts selected from the monofluorophosphatesalts and the difluorophosphate salts (the total amount when two or morekinds of the salts are used) may be 0.001 mass % or above, preferably0.01 mass % or above, more preferably 0.1 mass % or above, still morepreferably 0.2 mass % or above, and particularly preferably 0.3 mass %or above, and may be 5 mass % or less, preferably 3 mass % or less, morepreferably 2 mass % or less, still more preferably 1.5 mass % or less,and particularly preferably 1 mass % or less. This amount ensures thatthe salts produce significant effects in the enhancement of initialirreversible capacity.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and one or more selected from the monofluorophosphate saltsand the difluorophosphate salts (the total mass when two or more kindsof the salts are used) is preferably 1:99 to 99:1, more preferably 10:90to 90:10, and particularly preferably 20:80 to 80:20. This ratio ensuresthat the target characteristics may be enhanced without causing adecrease in other battery characteristics.

1-4-2. Borate Salts

The borate salts are not particularly limited as long as the salts haveat least one boron atom in the molecule. Those salts that correspond tooxalate salts are not categorized as the borate salts (1-4-2.) but arecategorized as the oxalate salts (1-4-3.) described later. In theelectrolytic solution of the invention, the combined use of the aromaticcarboxylate ester of Formula (2) with the borate salt results inimprovements in initial characteristics and storage characteristics, andalso enhances the overcharge safety of batteries.

Examples of the counter cations in the borate salts include lithium,sodium, potassium, magnesium, calcium, rubidium, cesium and barium, withlithium being preferable.

Preferred borate salts are lithium salts. Lithium borate-containingsalts may be also suitably used. Examples include LiBF₄, LiBF₃CF₃,LiBF₃C₂F₅, LiBF₃C₃F₇, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂(CF₃SO₂)₂ andLiBF₂(C₂F₅SO₂)₂. In particular, LiBF₄ is more preferable because of itseffect of enhancing characteristics such as initial charge-dischargeefficiency and high-temperature cycle characteristics.

The borate salts may be used singly, or two or more may be used incombination in an appropriate ratio.

The amount of the borate salt (the total amount when two or more kindsof the salts are used) may be 0.05 mass % or above, preferably 0.1 mass% or above, more preferably 0.2 mass % or above, still more preferably0.3 mass % or above, and particularly preferably 0.4 mass % or above,and may be 10.0 mass % or less, preferably 5.0 mass % or less, morepreferably 3.0 mass % or less, still more preferably 2.0 mass % or less,and particularly preferably 1.0 mass % or less. This amount ensures thatside reactions on the negative electrode are suppressed and the increasein battery resistance is unlikely to occur.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the borate salt is preferably 1:99 to 99:1, morepreferably 10:90 to 90:10, and particularly preferably 20:80 to 80:20.This ratio ensures that side reactions on the positive and negativeelectrodes in batteries are suppressed and the increase in batteryresistance is unlikely to occur.

When the borate salt and LiPF₆ are used as the electrolytes, the ratioof the molar content of the borate salt to the molar content of LiPF₆ inthe nonaqueous electrolytic solution is preferably 0.001 to 12, morepreferably 0.01 to 1.1, still more preferably 0.01 to 1.0, and furtherpreferably 0.01 to 0.7. This ratio ensures that side reactions on thepositive and negative electrodes in batteries will be prevented and thecharge discharge efficiency of batteries will be enhanced.

1-4-3. Oxalate Salts

The oxalate salts are not particularly limited as long as the compoundshave at least one oxalate structure in the molecule. In the electrolyticsolution of the invention, the combined use of the aromatic carboxylateester represented by Formula (2) and the oxalate salt results inbatteries enhanced in initial characteristics and storagecharacteristics.

Preferred oxalate salts are metal salts represented by Formula (9) belowwhich have an oxalate complex as the anion.

[Chem. 47]

M¹ _(a)[M²(C₂O₄)_(b)R_(c) ⁹¹]_(d)  (9)

(In the formula,

M¹ is an element selected from the group consisting of Group 1 and Group2 in the periodic table and aluminum (Al),

M² is an element selected from the group consisting of transitionmetals, and Group 13, Group 14 and Group 15 in the periodic table,

R⁹¹ is a group selected from the group consisting of halogens, alkylgroups having 1 to 11 carbon atoms and halogenated alkyl groups having 1to 11 carbon atoms,

a and b are positive integers,

c is 0 or a positive integer, and

d is an integer of 1 to 3.)

From the point of view of battery characteristics obtained when theelectrolytic solution of the invention is used for lithium secondarybatteries, M¹ is preferably lithium, sodium, potassium, magnesium orcalcium, and is particularly preferably lithium.

In terms of electrochemical stability in lithium secondary batteries, M²is particularly preferably boron or phosphorus.

Examples of R⁹¹ include fluorine, chlorine, methyl group,trifluoromethyl group, ethyl group, pentafluoroethyl group, propylgroup, isopropyl group, butyl group, sec-butyl group and tert-butylgroup, with fluorine and trifluoromethyl group being preferable.

Examples of the metal salts represented by Formula (9) include thefollowing:

lithium oxalatoborate salts such as lithium difluorooxalatoborate andlithium bis(oxalato)borate; and

lithium oxalatophosphate salts such as lithiumtetrafluorooxalatophosphate, lithium difluorobis(oxalato)phosphate andlithium tris(oxalato)phosphate.

Of these, lithium bis(oxalato)borate and lithiumdifluorobis(oxalato)phosphate are preferable, and lithiumbis(oxalato)borate is more preferable.

The oxalate salts may be used singly, or two or more may be used incombination in an appropriate ratio.

The amount of the oxalate salt (the total amount when two or more kindsof the salts are used) may be 0.001 mass % or above, preferably 0.01mass % or above, more preferably 0.1 mass % or above, and particularlypreferably 0.3 mass % or above, and may be 10 mass % or less, preferably5 mass % or less, more preferably 3 mass % or less, still morepreferably 2 mass % or less, and particularly preferably 1 mass % orless. This amount ensures easy control of characteristics such as outputcharacteristics, load characteristics, low-temperature characteristics,cycle characteristics and high-temperature storage characteristics.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the oxalate salt is preferably 1:99 to 99:1, morepreferably 10:90 to 90:10, and particularly preferably 20:80 to 80:20.This ratio ensures that side reactions on the positive and negativeelectrodes of batteries are suppressed with a good balance, and batterycharacteristics are enhanced easily.

1-4-4. Fluorosulfonate Salts

The fluorosulfonate salts are not particularly limited as long as thesalts have at least one fluorosulfonate structure in the molecule. Inthe electrolytic solution of the invention, the combined use of thearomatic carboxylate ester represented by Formula (2) and thefluorosulfonate salt results in batteries enhanced in initialcharacteristics and storage characteristics.

The counter cations in the fluorosulfonate salts are not particularlylimited. Examples thereof include lithium, sodium, potassium, rubidium,cesium, magnesium, calcium, barium and ammonium represented byNR¹³¹R¹³²R¹³³R¹³⁴ (wherein R¹³¹ to R¹³⁴ are independently a hydrogenatom or an organic group having 1 to 12 carbon atoms). Examples andpreferred examples of R¹³¹ to R¹³⁴ are similar to those of R¹²¹ to R¹²⁴described in 1-4-1. Preferred counter cations are lithium, sodium andpotassium. Lithium is particularly preferable.

Examples of the fluorosulfonate salts include lithium fluorosulfonate,sodium fluorosulfonate, potassium fluorosulfonate, rubidiumfluorosulfonate and cesium fluorosulfonate, with lithium fluorosulfonatebeing preferable. Imide salts having a fluorosulfonate structure such aslithium bis(fluorosulfonyl)imide may also be used as the fluorosulfonatesalts.

The fluorosulfonate salts may be used singly, or two or more may be usedin combination in an appropriate ratio.

The content of the fluorosulfonate salt (the total content when two ormore kinds of the salts are used) may be 0.05 mass % or above,preferably 0.1 mass % or above, more preferably 0.2 mass % or above,still more preferably 0.3 mass % or above, and particularly preferably0.4 mass % or above, and may be 10 mass % or less, preferably 8 mass %or less, more preferably 5 mass % or less, still more preferably 2 mass% or less, and particularly preferably 1 mass % or less. This contentensures that the occurrence of side reactions in batteries is reducedand the increase in resistance is unlikely to occur.

The mass ratio between the aromatic carboxylate ester represented byFormula (2) and the fluorosulfonate salt is preferably 1:99 to 99:1,more preferably 10:90 to 90:10, and particularly preferably 20:80 to80:20. This ratio ensures that side reactions in batteries areappropriately suppressed and the decrease in high-temperature durabilitycharacteristics is unlikely to occur.

1-5. Nonaqueous Solvents

The nonaqueous solvents in the present invention are not particularlylimited, and any known organic solvents may be used. Specific examplesinclude fluorine-free cyclic carbonates, chain carbonates, cyclic andchain carboxylate esters, ether compounds and sulfone compounds.

In the specification, the volumes of the nonaqueous solvents are valuesmeasured at 25° C. For those solvents which are solid at 25° C. such asethylene carbonate, volumes measured at the melting point are used.

1-5-1. Fluorine-Free Cyclic Carbonates

Examples of the fluorine-free cyclic carbonates include cycliccarbonates having an alkylene group with 2 to 4 carbon atoms.

Specific examples of the fluorine-free cyclic carbonates having analkylene group with 2 to 4 carbon atoms include ethylene carbonate,propylene carbonate and butylene carbonate. Of these, ethylene carbonateand propylene carbonate are particularly preferable because using thesesolvents enhances the degree of the dissociation of lithium ions andresults in an enhancement in battery characteristics.

The fluorine-free cyclic carbonates may be used singly, or two or moremay be used in combination in an appropriate ratio.

The amount of the fluorine-free cyclic carbonates is not particularlylimited and may be determined appropriately as long as the advantageouseffects of the present invention are not significantly impaired. Whenused singly, the amount of the carbonate is 5 vol % or above, and morepreferably 10 vol % or above in 100 vol % of the nonaqueous solvent.This amount makes it possible to avoid a decrease in electricconductivity due to the dielectric constant of the nonaqueouselectrolytic solution being low, and makes it easy for nonaqueouselectrolyte secondary batteries to achieve good characteristics such ashigh-current discharge characteristics, stability on negative electrodesand cycle characteristics. Further, the volume is 95 vol % or less, morepreferably 90 vol % or less, and still more preferably 85 vol % or less.This amount ensures that the nonaqueous electrolytic solution willexhibit an appropriate viscosity to prevent the decrease in ionconductivity, and that the nonaqueous electrolyte secondary batterieswill achieve good load characteristics.

1-5-2. Chain Carbonates

Preferred chain carbonates are those chain carbonates having 3 to 7carbon atoms. Dialkyl carbonates having 3 to 7 carbon atoms are morepreferable.

Examples of the chain carbonates include dimethyl carbonate, diethylcarbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propylisopropyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate,n-butyl methyl carbonate, isobutyl methyl carbonate, tert-butyl methylcarbonate, ethyl-n-propyl carbonate, n-butyl ethyl carbonate, isobutylethyl carbonate and tert-butyl ethyl carbonate.

Of these, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate,diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methylcarbonate and methyl-n-propyl carbonate are preferable. Dimethylcarbonate, diethyl carbonate and ethyl methyl carbonate are particularlypreferable.

Further, chain carbonates having a fluorine atom (hereinafter, alsowritten as “fluorinated chain carbonates”) may also be suitably used.

The fluorinated chain carbonates may have one or more fluorine atomswithout limitation. The number of fluorine atoms is usually 6 or less,and preferably 4 or less. When the fluorinated chain carbonate has aplurality of fluorine atoms, the fluorine atoms may be bonded to thesame carbon atom or to different carbon atoms.

Examples of the fluorinated chain carbonates include fluorinateddimethyl carbonate and derivatives thereof, fluorinated ethyl methylcarbonate and derivatives thereof, and fluorinated diethyl carbonate andderivatives thereof.

Examples of the fluorinated dimethyl carbonate and the derivativesthereof include fluoromethyl methyl carbonate, difluoromethyl methylcarbonate, trifluoromethyl methyl carbonate, bis(fluoromethyl)carbonate, bis(difluoromethyl) carbonate and bis(trifluoromethyl)carbonate.

Examples of the fluorinated ethyl methyl carbonate and the derivativesthereof include 2-fluoroethyl methyl carbonate, ethyl fluoromethylcarbonate, 2,2-difluoroethyl methyl carbonate, 2-fluoroethylfluoromethyl carbonate, ethyl difluoromethyl carbonate,2,2,2-trifluoroethyl methyl carbonate, 2,2-difluoroethyl fluoromethylcarbonate, 2-fluoroethyl difluoromethyl carbonate and ethyltrifluoromethyl carbonate.

Examples of the fluorinated diethyl carbonate and the derivativesthereof include ethyl-(2-fluoroethyl) carbonate,ethyl-(2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate,ethyl-(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl-2′-fluoroethylcarbonate, bis(2,2-difluoroethyl) carbonate,2,2,2-trifluoroethyl-2′-fluoroethyl carbonate,2,2,2-trifluoroethyl-2′,2′-difluoroethyl carbonate andbis(2,2,2-trifluoroethyl) carbonate.

The chain carbonates may be used singly, or two or more may be used incombination in an appropriate ratio. In 100 vol % of the nonaqueoussolvent, the amount of the chain carbonate(s) is preferably 5 vol % orabove, more preferably 10 vol % or above, and still more preferably 15vol % or above. This lower limit ensures that the nonaqueouselectrolytic solution exhibits an appropriate viscosity and thus thedecrease in ion conductivity is prevented, making it easy for nonaqueouselectrolyte secondary batteries to achieve excellent high-currentdischarge characteristics. Further, the volume of the chain carbonate(s)in 100 vol % of the nonaqueous solvent is preferably 90 vol % or less,and more preferably 85 vol % or less. This upper limit makes it possibleto avoid a decrease in electric conductivity due to the dielectricconstant of the nonaqueous electrolytic solution being low, and makes iteasy for nonaqueous electrolyte secondary batteries to achieve excellenthigh-current discharge characteristics.

1-5-3. Cyclic Carboxylate Esters

Preferred cyclic carboxylate esters are those having 3 to 12 carbonatoms.

Specific examples include gamma-butyrolactone, gamma-valerolactone,gamma-caprolactone and epsilon-caprolactone. Of these,gamma-butyrolactone is particularly preferable because the use thereofenhances the degree of the dissociation of lithium ions and results inan enhancement in battery characteristics.

The cyclic carboxylate esters may be used singly, or two or more may beused in combination in an appropriate ratio.

In 100 vol % of the nonaqueous solvent, the amount of the cycliccarboxylate ester(s) is preferably 5 vol % or above, and more preferably10 vol % or above. This amount ensures that the electric conductivity ofthe nonaqueous electrolytic solution is improved to make it easy fornonaqueous electrolyte secondary batteries to achieve an enhancement inhigh-current discharge characteristics. The amount of the cycliccarboxylate ester(s) is preferably 50 vol % or less, and more preferably40 vol % or less. This upper limit ensures that the nonaqueouselectrolytic solution exhibits an appropriate viscosity, and thedecrease in electric conductivity is avoided and the increase innegative electrode resistance is suppressed, making it easy fornonaqueous electrolyte secondary batteries to achieve excellenthigh-current discharge characteristics.

1-5-4. Ether Compounds

Preferred ether compounds are chain ethers having 3 to 10 carbon atomsand cyclic ethers having 3 to 6 carbon atoms. Part of the hydrogen atomsin the ether compounds may be substituted by fluorine atoms.

Examples of the chain ethers having 3 to 10 carbon atoms include:

diethyl ether, di(2-fluoroethyl) ether, di(2,2-difluoroethyl) ether,di(2,2,2-trifluoroethyl) ether, ethyl (2-fluoroethyl) ether, ethyl(2,2,2-trifluoroethyl) ether, ethyl (1,1,2,2-tetrafluoroethyl) ether,(2-fluoroethyl) (2,2,2-trifluoroethyl) ether, (2-fluoroethyl)(1,1,2,2-tetrafluoroethyl) ether, (2,2,2-trifluoroethyl)(1,1,2,2-tetrafluoroethyl) ether, ethyl-n-propyl ether, ethyl(3-fluoro-n-propyl) ether, ethyl (3,3,3-trifluoro-n-propyl) ether, ethyl(2,2,3,3-tetrafluoro-n-propyl) ether, ethyl(2,2,3,3,3-pentafluoro-n-propyl) ether, 2-fluoroethyl-n-propyl ether,(2-fluoroethyl) (3-fluoro-n-propyl) ether, (2-fluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, 2,2,2-trifluoroethyl-n-propylether, (2,2,2-trifluoroethyl) (3-fluoro-n-propyl) ether,(2,2,2-trifluoroethyl) (3,3,3-trifluoro-n-propyl) ether,(2,2,2-trifluoroethyl) (2,2,3,3-tetrafluoro-n-propyl) ether,(2,2,2-trifluoroethyl) (2,2,3,3,3-pentafluoro-n-propyl) ether,1,1,2,2-tetrafluoroethyl-n-propyl ether, (1,1,2,2-tetrafluoroethyl)(3-fluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di-n-propyl ether, (n-propyl)(3-fluoro-n-propyl) ether, (n-propyl) (3,3,3-trifluoro-n-propyl) ether,(n-propyl) (2,2,3,3-tetrafluoro-n-propyl) ether, (n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3-fluoro-n-propyl) ether,(3-fluoro-n-propyl) (3,3,3-trifluoro-n-propyl) ether,(3-fluoro-n-propyl) (2,2,3,3-tetrafluoro-n-propyl) ether,(3-fluoro-n-propyl) (2,2,3,3,3-pentafluoro-n-propyl) ether,di(3,3,3-trifluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(2,2,3,3-tetrafluoro-n-propyl)ether, (2,2,3,3-tetrafluoro-n-propyl) (2,2,3,3,3-pentafluoro-n-propyl)ether, di(2,2,3,3,3-pentafluoro-n-propyl) ether, di-n-butyl ether,dimethoxymethane, methoxyethoxymethane, methoxy(2-fluoroethoxy)methane,methoxy(2,2,2-trifluoroethoxy)methane,methoxy(1,1,2,2-tetrafluoroethoxy)methane, diethoxymethane,ethoxy(2-fluoroethoxy)methane, ethoxy(2,2,2-trifluoroethoxy)methane,ethoxy(1,1,2,2-tetrafluoroethoxy)methane, di(2-fluoroethoxy)methane,(2-fluoroethoxy)(2,2,2-trifluoroethoxy)methane,(2-fluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane,di(2,2,2-trifluoroethoxy)methane,(2,2,2-trifluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane,di(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane,methoxyethoxyethane, methoxy(2-fluoroethoxy)ethanc,methoxy(2,2,2-trifluoroethoxy)ethane,methoxy(1,1,2,2-tetrafluoroethoxy)ethane, diethoxyethane,ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane,ethoxy(1,1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane,(2-fluoroethoxy)(2,2,2-trifluoroethoxy)ethane,(2-fluoroethoxy)(1,1,2,2-tetrafluoroethoxy)ethane,di(2,2,2-trifluoroethoxy)ethane,(2,2,2-trifluoroethoxy)(1,1,2,2-tetrafluoroethoxy)ethane,di(1,1,2,2-tetrafluoroethoxy)ethane, ethylene glycol di-n-propyl ether,ethylene glycol di-n-butyl ether and diethylene glycol dimethyl ether.

Examples of the cyclic ethers having 3 to 6 carbon atoms includetetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran,1,4-dioxane and fluorides of these compounds.

In particular, dimethoxymethane, diethoxymethane, ethoxymethoxymethane,ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether anddiethylene glycol dimethyl ether are preferable because of their highabilities to solvate lithium ions and to enhance the dissociation of theions. Dimethoxymethane, diethoxymethane and ethoxymethoxymethane areparticularly preferable because these solvents exhibit a low viscosityand provide high ion conductivity.

The ether compounds may be used singly, or two or more may be used incombination in an appropriate ratio.

In 100 vol % of the nonaqueous solvent, the amount of the ethercompound(s) is preferably 5 vol % or above, more preferably 10 vol % orabove, and still more preferably 15 vol % or above, and is preferably 70vol % or less, more preferably 60 vol % or less, and still morepreferably 50 vol % or less. This amount ensures that the ionconductivity will be enhanced due to the enhancement of the degree oflithium ion dissociation and the reduction in viscosity by virtue of theuse of the chain ether. Further, the above amount ensures that in thecase where the negative electrode active material is a carbonaceousmaterial, the decrease in capacity due to the co-intercalation of thechain ether together with lithium ions will be avoided.

1-5-5. Sulfone Compounds

Preferred sulfone compounds are cyclic sulfones having 3 to 6 carbonatoms, and chain sulfones having 2 to 6 carbon atoms. The number of thesulfonyl groups in the molecule is preferably 1 or 2.

Examples of the cyclic sulfones having 3 to 6 carbon atoms includemonosulfone compounds such as trimethylenesulfones,tetramethylenesulfones and hexamethylenesulfones; and

disulfone compounds such as trimethylenedisulfones,tetramethylenedisulfones and hexamethylenedisulfones.

From the points of view of dielectric constant and viscosity,tetramethylenesulfones, tetramethylenedisulfones, hexamethylenesulfonesand hexamethylenedisulfones are more preferable, andtetramethylenesulfones (sulfolanes) are particularly preferable.

Preferred sulfolanes are sulfolane and/or sulfolane derivatives(hereinafter, derivatives including sulfolane itself are sometimeswritten as “sulfolanes”). Preferred sulfolane derivatives are those inwhich one or more hydrogen atoms bonded to the carbon atoms constitutingthe sulfolane ring are substituted by fluorine atoms or alkyl groups.

In particular, some preferred sulfolanes having high ion conductivityand realizing high input and output characteristics are2-methylsulfolane, 3-methylsulfolane, 2-fluorosulfolane,3-fluorosulfolane, 2,2-difluorosulfolane, 2,3-difluorosulfolane,2,4-difluorosulfolane, 2,5-difluorosulfolane, 3,4-difluorosulfolane,2-fluoro-3-methylsulfolane, 2-fluoro-2-methylsulfolane,3-fluoro-3-methylsulfolane, 3-fluoro-2-methylsulfolane,4-fluoro-3-methylsulfolane, 4-fluoro-2-methylsulfolane,5-fluoro-3-methylsulfolane, 5-fluoro-2-methylsulfolane,2-fluoromethylsulfolane, 3-fluoromethylsulfolane,2-difluoromethylsulfolane, 3-difluoromethylsulfolane,2-trifluoromethyisulfolane, 3-trifluoromethylsulfolane,2-fluoro-3-(trifluoromethyl)sulfolane,3-fluoro-3-(trifluoromethyl)sulfolane,4-fluoro-3-(trifluoromethyl)sulfolane and5-fluoro-3-(trifluoromethyl)sulfolane.

Examples of the chain sulfones having 2 to 6 carbon atoms include:

dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, n-propyl methylsulfone, n-propyl ethyl sulfone, di-n-propyl sulfone, isopropyl methylsulfone, isopropyl ethyl sulfone, diisopropyl sulfone, n-butyl methylsulfone, n-butyl ethyl sulfone, tert-butyl methyl sulfone, tert-butylethyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methylsulfone, trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone,difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone,pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone, ethyldifluoromethyl sulfone, ethyl trifluoromethyl sulfone, perfluoroethylmethyl sulfone, ethyl trifluoroethyl sulfone, ethyl pentafluoroethylsulfone, di(trifluoroethyl) sulfone, perfluorodiethyl sulfone,fluoromethyl-n-propyl sulfone, difluoromethyl-n-propyl sulfone,trifluoromethyl-n-propyl sulfone, fluoromethyl isopropyl sulfone,difluoromethyl isopropyl sulfone, trifluoromethyl isopropyl sulfone,trifluoroethyl-n-propyl sulfone, trifluoroethyl isopropyl sulfone,pentafluoroethyl-n-propyl sulfone, pentafluoroethyl isopropyl sulfone,trifluoroethyl-n-butyl sulfone, trifluoroethyl-tert-butyl sulfone,pentafluoroethyl-n-butyl sulfone and pentafluoroethyl-tert-butylsulfone.

In particular, some preferred sulfones having high ion conductivity andrealizing high input and output characteristics are dimethyl sulfone,ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone,isopropyl methyl sulfone, n-butyl methyl sulfone, tert-butyl methylsulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone,trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone,difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone,pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone, ethyldifluoromethyl sulfone, ethyl trifluoromethyl sulfone, ethyltrifluoroethyl sulfone, ethyl pentafluoroethyl sulfone,trifluoromethyl-n-propyl sulfone, trifluoromethyl isopropyl sulfone,trifluoroethyl-n-butyl sulfone, trifluoroethyl-tert-butyl sulfone,trifluoromethyl-n-butyl sulfone and trifluoromethyl-tert-butyl sulfone.

The sulfone compounds may be used singly, or two or more may be used incombination in an appropriate ratio.

In 100 vol % of the nonaqueous solvent, the amount of the sulfonecompound(s) is preferably 0.3 vol % or above, more preferably 1 vol % orabove, and still more preferably 5 vol % or above, and is preferably 40vol % or less, more preferably 35 vol % or less, and still morepreferably 30 vol % or less. This amount ensures that durability such ascycle characteristics and storage characteristics will be enhanced, andthat the nonaqueous electrolytic solution exhibits an appropriateviscosity to make it possible to avoid a decrease in electricconductivity. Thus, nonaqueous electrolyte secondary batteries may becharged and discharged with a high current density while avoiding adecrease in the retention of charge and discharge capacities.

1-5-6. Compositions of Nonaqueous Solvents

The nonaqueous solvent in the invention may be any one solvent selectedfrom the nonaqueous solvents described above, or may be a combination oftwo or more kinds of the solvents in an appropriate ratio.

For example, a preferred combination of the nonaqueous solvents is onebased on a fluorine-free cyclic carbonate and a chain carbonate.

In particular, the total of the fluorine-free cyclic carbonate and thechain carbonate is preferably 70 vol % or above, more preferably 80 vol% or above, and still more preferably 90 vol % or above of the wholenonaqueous solvent, and the proportion of the fluorine-free cycliccarbonate to the total of the cyclic carbonate and the chain carbonateis preferably 5 vol % or above, more preferably 10 vol % or above, andstill more preferably 15 vol % or above, and is preferably 50 vol % orless, more preferably 35 vol % or less, still more preferably 30 vol %or less, and particularly preferably 25 vol % or less.

In some cases, the use of this combination of the nonaqueous solventsresults in a good balance between cycle characteristics andhigh-temperature storage characteristics (in particular, retention ofcapacity after storage at high temperatures, and high-load dischargecapacity) of batteries manufactured with the nonaqueous solvents.

Preferred examples of the combinations of the fluorine-free cycliccarbonates and the chain carbonates include:

ethylene carbonate and dimethyl carbonate; ethylene carbonate anddiethyl carbonate; ethylene carbonate and ethyl methyl carbonate;ethylene carbonate, dimethyl carbonate and diethyl carbonate; ethylenecarbonate, dimethyl carbonate and ethyl methyl carbonate; ethylenecarbonate, diethyl carbonate and ethyl methyl carbonate; and ethylenecarbonate, dimethyl carbonate, diethyl carbonate and ethyl methylcarbonate.

Of the combinations of the fluorine-free cyclic carbonates and the chaincarbonates, those which include an asymmetric chain alkyl carbonate asthe chain carbonate are more preferable. In particular, those whichinclude ethylene carbonate, a symmetric chain carbonate and anasymmetric chain carbonate are preferable because a good balance isobtained between cycle characteristics and high-current dischargecharacteristics, with examples of such combinations including ethylenecarbonate, dimethyl carbonate and ethyl methyl carbonate; ethylenecarbonate, diethyl carbonate and ethyl methyl carbonate; and ethylenecarbonate, dimethyl carbonate, diethyl carbonate and ethyl methylcarbonate.

In particular, those combinations in which the asymmetric chaincarbonate is ethyl methyl carbonate are preferable, and the alkyl groupsin the chain carbonate preferably have 1 to 2 carbon atoms.

Examples of the preferred combinations further include thosecombinations including propylene carbonate in the aforementionedcombinations of ethylene carbonate and the chain carbonate(s).

When propylene carbonate is used, the volume ratio between ethylenecarbonate and propylene carbonate is preferably 99:1 to 40:60, andparticularly preferably 95:5 to 50:50. Further, the proportion ofpropylene carbonate in the whole nonaqueous solvent is preferably 0.1vol % or above, more preferably 1 vol % or above, and still morepreferably 2 vol % or above, and is preferably 20 vol % or less, morepreferably 8 vol % or less, and still more preferably 5 vol % or less.

This concentration of propylene carbonate is advantageous in thatlow-temperature characteristics may be further enhanced at times whilemaintaining the characteristics obtained by the combination of ethylenecarbonate and the chain carbonate.

When the nonaqueous solvent includes dimethyl carbonate, the proportionof dimethyl carbonate in the whole nonaqueous solvent is preferably 10vol % or above, more preferably 20 vol % or above, still more preferably25 vol % or above, and particularly preferably 30 vol % or above, and ispreferably 90 vol % or less, more preferably 80 vol % or less, stillmore preferably 75 vol % or less, and particularly preferably 70 vol %or less. This concentration leads to an enhancement in loadcharacteristics of batteries at times.

In particular, the use of a nonaqueous solvent which includes dimethylcarbonate and ethyl methyl carbonate and in which the content ofdimethyl carbonate is higher than the content of ethyl methyl carbonateis preferable because battery characteristics after high-temperaturestorage are enhanced at times while maintaining the electricconductivity of the electrolytic solution.

In the whole nonaqueous solvent, the volume ratio of dimethyl carbonateto ethyl methyl carbonate (dimethyl carbonate/ethyl methyl carbonate) ispreferably 1.1 or above, more preferably 1.5 or above, and still morepreferably 2.5 or above in order to enhance the electric conductivity ofthe electrolytic solution and battery characteristics after storage. Theabove volume ratio (dimethyl carbonate/ethyl methyl carbonate) ispreferably 40 or less, more preferably 20 or less, still more preferably10 or less, and particularly preferably 8 or less in order to enhancebattery characteristics at low temperatures.

The combinations based on the fluorine-free cyclic carbonates and thechain carbonates may include other solvents such as cyclic carboxylateesters, chain carboxylate esters, cyclic ethers, chain ethers,sulfur-containing organic solvents, phosphorus-containing organicsolvents and fluorine-containing aromatic solvents.

1-6. Auxiliaries

In the electrolyte batteries according to the invention, auxiliaries maybe used appropriately in accordance with the purpose in addition to thecompounds described hereinabove. Examples of the auxiliaries includecyclic carbonates having a carbon-carbon unsaturated bond describedbelow, and other auxiliaries described later.

1-6-1. Cyclic Carbonates Having Carbon-Carbon Unsaturated Bond

The cyclic carbonates having a carbon-carbon unsaturated bond(hereinafter, also written as “unsaturated cyclic carbonates”) are notparticularly limited and any unsaturated carbonates may be used as longas the cyclic carbonates have a carbon-carbon double bond or acarbon-carbon triple bond. Those cyclic carbonates which have anaromatic ring are also categorized as the unsaturated cyclic carbonates.

Examples of the unsaturated cyclic carbonates include vinylenecarbonates, ethylene carbonates substituted with a substituent having anaromatic ring or a carbon-carbon double or triple bond, phenylcarbonates, vinyl carbonates, allyl carbonates and catechol carbonates.

Examples of the vinylene carbonates include vinylene carbonate,methylvinylene carbonate, 4,5-dimethylvinylene carbonate, phenylvinylenecarbonate, 4,5-diphenylvinylene carbonate, vinylvinylene carbonate,4,5-divinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, 4-fluorovinylene carbonate,4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-phenylvinylenecarbonate, 4-fluoro-5-vinylvinylene carbonate and4-allyl-5-fluorovinylene carbonate.

Specific examples of the ethylene carbonates substituted with asubstituent having an aromatic ring or a carbon-carbon double or triplebond include vinylethylene carbonate, 4,5-divinylethylene carbonate,4-methyl-5-vinylethylene carbonate, 4-allyl-5-vinylethylene carbonate,ethynylethylene carbonate, 4,5-diethynylethylene carbonate,4-methyl-5-ethynylethylene carbonate, 4-vinyl-5-ethynylethylenecarbonate, 4-allyl-5-ethynylethylene carbonate, phenylethylenecarbonate, 4,5-diphenylethylene carbonate, 4-phenyl-5-vinylethylenecarbonate, 4-allyl-5-phenylethylene carbonate, allylethylene carbonate,4,5-diallylethylene carbonate and 4-methyl-5-allylethylene carbonate.

Of these, unsaturated cyclic carbonates which are particularly suitedfor the combined use are vinylene carbonate, methylvinylene carbonate,4,5-dimethylvinylene carbonate, vinylvinylene carbonate,4,5-divinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, vinylethylene carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, 4-allyl-5-vinylethylene carbonate,ethynylethylene carbonate, 4,5-diethynylethylene carbonate,4-methyl-5-ethynylethylene carbonate and 4-vinyl-5-ethynylethylenecarbonate. Vinylene carbonate, vinylethylene carbonate andethynylethylene carbonate are preferable because they form a highlystable interface protective film. Vinylene carbonate and vinylethylenecarbonate are more preferable.

The molecular weight of the unsaturated cyclic carbonates is notparticularly limited as long as the advantageous effects of theinvention are not significantly impaired. The molecular weight ispreferably 80 or above, and more preferably 85 or above, and ispreferably 250 or less, and more preferably 150 or less. This range ofmolecular weights ensures that the unsaturated cyclic carbonate willexhibit solubility with respect to the nonaqueous electrolytic solutionand the advantageous effects of the invention are obtained prominently.

The unsaturated cyclic carbonates may be produced by any methods withoutlimitation, and known production methods may be selected appropriately.

The unsaturated cyclic carbonates may be used singly, or two or more maybe used in combination in an appropriate ratio. The amount of theunsaturated cyclic carbonate(s) is not particularly limited and may bedetermined appropriately as long as the advantageous effects of theinvention are not significantly impaired. In 100 mass % of thenonaqueous electrolytic solution, the amount of the unsaturated cycliccarbonate(s) is preferably 0.001 mass % or above, more preferably 0.01mass % or above, and still more preferably 0.1 mass % or above, and ispreferably 10 mass % or less, more preferably 5 mass % or less, stillmore preferably 4 mass % or less, and particularly preferably 3 mass %or less. This amount ensures that the obtainable nonaqueous electrolytesecondary batteries will achieve a sufficient enhancement in cyclecharacteristics and also reduces the probability that high-temperaturestorage characteristics are decreased to cause a heavy generation of gasand a poor retention of discharge capacity.

1-6-2. Other Auxiliaries

The electrolytic solution of the invention may contain other knownauxiliaries. Examples of such additional auxiliaries include carbonatecompounds such as erythritan carbonate, spiro-bis-dimethylene carbonateand methoxyethyl-methyl carbonate; carboxylic anhydrides such assuccinic anhydride, glutaric anhydride, maleic anhydride, citraconicanhydride, glutaconic anhydride, itaconic anhydride, diglycolicanhydride, cyclohexanedicarboxylic anhydride,cyclopentanetetracarboxylic dianhydride and phenylsuccinic anhydride;spiro compounds such as 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane;sulfur-containing compounds such as N,N-dimethylmethanesulfonamide andN,N-diethylmethanesulfonamide; phosphorus-containing compounds such astrimethyl phosphite, triethyl phosphite, triphenyl phosphite, trimethylphosphate, triethyl phosphate, triphenyl phosphate, methyldimethylphosphinate, ethyl diethylphosphinate, trimethylphosphine oxideand triethylphosphine oxide; nitrogen-containing compounds such as1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone,3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone andN-methylsuccinimide; and hydrocarbon compounds such as heptane, octane,nonane, decane and cycloheptane. These auxiliaries may be used singly,or two or more may be used in combination. The addition of theseauxiliaries enhances the retention of capacity after high-temperaturestorage and cycle characteristics.

The amount of the additional auxiliaries is not particularly limited andmay be determined appropriately as long as the advantageous effects ofthe invention are not significantly impaired. In 100 mass % of thenonaqueous electrolytic solution, the amount of the additionalauxiliaries is preferably 0.01 mass % to 5 mass %. When added in thisamount, the additional auxiliaries will sufficiently produce theireffects while the decrease in battery characteristics such as high-loaddischarge characteristics will be avoided. The amount of the additionalauxiliaries is more preferably 0.1 mass % or above, and still morepreferably 0.2 mass % or above, and is more preferably 3 mass % or less,and still more preferably 1 mass % or less.

2. Battery Configurations

The electrolytic solution for nonaqueous electrolyte secondary batteriesaccording to the present invention is suitably used for secondarybatteries, for example, lithium secondary batteries. Hereinbelow,nonaqueous electrolyte secondary batteries involving the electrolyticsolution of the invention will be described.

The electrolyte batteries of the invention may have a known structure.Typically, the electrolyte batteries include a negative electrode and apositive electrode capable of storing and releasing metal ions (forexample, lithium ions), and the inventive electrolytic solutiondescribed above.

2-1. Negative Electrodes

Hereinbelow, negative electrode active materials used in the negativeelectrodes will be described. The negative electrode active materialsare not particularly limited as long as the materials canelectrochemically store and release metal ions (for example, lithiumions). Specific examples include carbonaceous materials, alloy materialsand lithium-metal composite oxide materials. These materials may be usedsingly, or two or more may be used in combination.

(Negative Electrode Active Materials)

Examples of the negative electrode active materials include carbonaceousmaterials, alloy materials and lithium-metal composite oxide materials.

In terms of the balance between initial irreversible capacity andhigh-current density charge/discharge characteristics, the carbonaceousmaterial used as the negative electrode active material is preferablyselected from:

(1) natural graphites;

(2) carbonaceous materials obtained by heat treating artificialcarbonaceous substances and artificial graphitic substances one or moretimes at 400 to 3200° C.;

(3) carbonaceous materials that form a negative electrode activematerial layer which is composed of at least two kinds of carbonaceoussubstances having different crystallinities and/or which has aninterface formed by such different crystalline carbonaceous substances;and

(4) carbonaceous materials that form a negative electrode activematerial layer which is composed of at least two kinds of carbonaceoussubstances having different orientations and/or which has an interfaceformed by such carbonaceous substances having different orientations.

The carbonaceous materials (1) to (4) may be used singly, or two or moremay be used in combination in an appropriate ratio.

Examples of the artificial carbonaceous substances and the artificialgraphitic substances used in (2) above include natural graphites, coalcokes, petroleum cokes, coal pitches, petroleum pitches, oxidationproducts of these pitches, needle cokes, pitch cokes, carbon materialsobtained by the partial graphitization of these cokes, furnace blacks,acetylene blacks, pyrolysates of organic substances such as pitch-basedcarbon fibers, carbonizable organic substances, carbides of suchsubstances, solutions of carbonizable organic substances inlow-molecular organic solvents such as benzene, toluene, xylene,quinoline and n-hexane, and carbides obtained from such solutions.

The alloy materials used as the negative electrode active materials arenot particularly limited as long as the materials can store and releaselithium, and may be any of elemental lithium, elemental metals andalloys capable of forming lithium alloys, and compounds of these metalssuch as oxides, carbides, nitrides, silicides, sulfides and phosphides.The elemental metals and the alloys capable of forming lithium alloysare preferably materials including Group 13 and Group 14 metals andsemimetals (except carbon), and are more preferably aluminum, siliconand tin (hereinafter, these metals are sometimes written as “specificmetal elements”), and alloys and compounds including atoms of theseelemental metals. These materials may be used singly, or two or more maybe used in combination in an appropriate ratio.

Examples of the negative electrode active materials having atoms of atleast one selected from the specific metal elements include individualelemental metals of the specific metal elements, alloys of two or moreof the specific metal elements, alloys of one, or two or more of thespecific metal elements and one, or two or more other metal elements,compounds containing one, or two or more of the specific metal elements,and composites such as oxides, carbides, nitrides, silicides, sulfidesand phosphides of the above compounds. The use of these elementalmetals, alloys or metal compounds as the negative electrode activematerials realizes a high capacity of the batteries.

Examples further include compounds in which the above composites formcomplex bonds with elemental metals, alloys or several elements such asnonmetal elements. Specific examples include alloys of silicon and/ortin, with a metal having no negative electrode action. For example, usemay be made of complex compounds which contain as many kinds of elementsas 5 to 6 including tin, a metal(s) other than tin and silicon thatserves as a negative electrode, a metal(s) having no negative electrodeaction, and a nonmetal element(s).

Of these negative electrode active materials, individual elementalmetals of the specific metal elements, alloys of two or more of thespecific metal elements, and compounds of the specific metal elementssuch as oxides, carbides and nitrides are preferable because theobtainable batteries exhibit a high capacity per unit mass. Inparticular, elemental silicon and/or tin, alloys of these elementalmetals, and compounds such as oxides, carbides and nitrides arepreferable from the points of view of the capacity per unit mass and theenvironmental load.

The lithium-metal composite oxide materials used as the negativeelectrode active materials are not particularly limited as long as thematerials can store and release lithium. From the point of view ofhigh-current density charge/discharge characteristics, those materialscontaining titanium and lithium are preferable, lithium-metal compositeoxide materials that contain titanium are more preferable, and compositeoxides of lithium and titanium (hereinafter, also written as“lithium-titanium composite oxides”) are still more preferable. That is,the use of the negative electrode active material including alithium-titanium composite oxide with a spinel structure is particularlypreferable in that the output resistance of the obtainable nonaqueouselectrolyte secondary batteries is significantly reduced.

It is also preferable that lithium and titanium in the lithium-titaniumcomposite oxide be substituted by other metal element, for example, atleast one element selected from the group consisting of Na, K, Co, Al,Fe, Ti, Mg, Cr, Ga, Cu, Zn and Nb.

It is preferable that the metal oxide be a lithium-titanium compositeoxide represented by Formula (A) and in Formula (A), 0.7≤x≤1.5,1.5≤y≤2.3, and 0≤z≤1.6. The structure of such an oxide is stable duringthe doping and the dedoping of lithium ions.

Li_(x)Ti_(y)M_(z)O₄  (A)

[In Formula (A), M is at least one element selected from the groupconsisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn and Nb.]

Of the compositions represented by Formula (A), those in which:

(a) 1.2≤x≤1.4, 1.5≤y≤1.7, and z=0,

(b) 0.9≤x≤1.1, 1.9≤y≤2.1, and z=0, or

(c) 0.7≤x≤0.9, 2.1≤y≤2.3, and z=0

are particularly preferable because a good balance in batteryperformances is obtained.

Particularly preferred typical compositions of the compounds areLi_(4/3)Ti_(5/3)O₄ for (a), Li₁Ti₂O₄ for (b), and Li_(4/5)Ti_(11/5)O₄for (c). Preferred examples of the structures in which Z≠0 includeLi_(4/3)Ti_(4/3)Al_(1/3)O₄.

(Properties of Carbonaceous Materials)

The carbonaceous materials which may be used as the negative electrodeactive materials preferably have the following properties.

(X-Ray Parameters)

The d value (the interlayer distance) between lattice planes (002planes) obtained by X-ray diffractometry according to GAKUSHIN methodwith respect to the carbonaceous material is preferably 0.335 nm ormore, and is usually 0.360 nm or less, preferably 0.350 nm or less, andmore preferably 0.345 nm or less. Further, the crystallite size (Lc) ofthe carbonaceous material determined by X-ray diffractometry accordingto GAKUSHIN method is preferably 1.0 nm or more, and more preferably 1.5nm or more.

(Volume-Based Average Particle Diameter)

The Volume-Based (Volume-Based Average Particle Diameter)

The mass-based average particle diameter of the carbonaceous material isthe average particle diameter (the median diameter) on volume basisdetermined by a laser diffraction/scattering method, and is usually 1 μmor more, preferably 3 μm or more, still more preferably 5 μm or more,and particularly preferably 7 μm or more, and is usually 100 μm or less,preferably 50 μm or less, more preferably 40 μm or less, still morepreferably 30 μm or less, and particularly preferably 25 μm or less.

If the volume-based average particle diameter is below the above range,the irreversible capacity is so increased that the batteries may sufferan initial capacity loss at times. In terms of battery production steps,any average particle diameter exceeding the above range is not desirableat times because the application of an electrode-forming liquidcontaining such particles tends to result in uneven coatings.

The volume-based average particle diameter is measured in such a mannerthat the carbon powder is dispersed in a 0.2 mass % aqueous solution(approximately 10 mL) of polyoxyethylene (20) sorbitan monolaurate as asurfactant, and the dispersion is analyzed with a laserdiffraction/scattering grain size distribution analyzer (LA-700manufactured by Horiba, Ltd.).

(Raman R Value and Raman Half Width)

The Raman R value of the carbonaceous material is a value measured by anargon ion laser Raman spectroscopy method, and is usually 0.01 or more,preferably 0.03 or more, and more preferably 0.1 or more, and is usually1.5 or less, preferably 1.2 or less, more preferably 1 or less, andparticularly preferably 0.5 or less.

The Raman half width at near 1580 cm⁻¹ of the carbonaceous material is,although not particularly limited to, usually 10 cm⁻¹ or more, andpreferably 15 cm⁻¹ or more, and is usually 100 cm⁻¹ or less, preferably80 cm⁻¹ or less, more preferably 60 cm⁻¹ or less, and particularlypreferably 40 cm⁻¹ or less.

The Raman R value and the Raman half width are indexes that indicate thecrystallinity of the surface of the carbonaceous material. It ispreferable that the carbonaceous material have appropriate crystallinityfrom the point of view of chemical stability, and that the crystallinitystill provide interlayer sites for the intercalation and thedeintercalation of lithium during charging and discharging, in otherwords, charge acceptability be not decreased. It is preferable to takeinto consideration the fact that when the carbonaceous material appliedon a current collector is pressed to increase the density of thenegative electrode, the crystals tend to be oriented in a directionparallel to the electrode plate. When the Raman R value or the Ramanhalf width is in the above range, the crystals allow a good film to beformed on the surface of the negative electrode and thus make itpossible to enhance storage characteristics, cycle characteristics andload characteristics and also make it possible to prevent the decreasein efficiency and the generation of gas associated with the reactionwith the nonaqueous electrolytic solution.

A Raman spectrum is obtained using a Raman spectrometer (RamanSpectrometer manufactured by JASCO Corporation) in such a manner thatthe sample is allowed to fall into the measurement cell to fill the celland the sample is analyzed while applying an argon ion laser beam to thesurface of the sample in the cell and while rotating the cell in a planeperpendicular to the laser beam. With respect to the Raman spectrumobtained, the intensity IA of a peak PA near 1580 cm⁻¹ and the intensityIB of a peak PB near 1360 cm⁻¹ are measured and the ratio R of theintensities (R=IB/IA) is calculated.

The Raman measurement conditions are as follows:

-   -   Argon ion laser wavelength: 514.5 nm    -   Laser power on sample: 15 to 25 mW    -   Resolution: 10 to 20 cm⁻¹    -   Measurement range: 1100 cm⁻¹ to 1730 cm⁻¹    -   Raman R value and Raman half width analysis: background        processing    -   Smoothing processing: simple averaging, convolution 5 points

(BET Specific Surface Area)

The BET specific surface area of the carbonaceous material is a value ofspecific surface area measured by a BET method, and is usually 0.1m²·g⁻¹ or more, preferably 0.7 m²·g⁻¹ or more, more preferably 1.0m²·g⁻¹ or more, and particularly preferably 1.5 m²·g⁻¹ or more, and isusually 100 m²·g⁻¹ or less, preferably 25 m²·g⁻¹ or less, morepreferably 15 m²·g⁻¹ or less, and particularly preferably 10 m²·g⁻¹ orless.

When the BET specific surface area is in the above range, theprecipitation of lithium on the electrode surface can be prevented whilethe generation of gas by the reaction with the nonaqueous electrolyticsolution can be suppressed.

The specific surface area is measured by the BET method using a surfacearea meter (an automatic surface area measuring apparatus manufacturedby Okura Riken) in such a manner that the sample is preliminarily driedat 350° C. under a stream of nitrogen for 15 minutes and thereafter theanalysis is performed by the nitrogen adsorption BET single point methodby flowing a nitrogen-helium mixed gas prepared so that the pressure ofnitrogen relative to the atmospheric pressure is exactly 0.3.

(Circularity)

The circularity which indicates how close the carbonaceous material isto a sphere is preferably in the range described below. The circularityis defined as “Circularity=(Circumferential length of equivalent circlehaving equal area to projection of particle)/(Actual circumferentiallength of projection of particle)”. When the circularity is 1, theparticle is theoretically spherical. For particles of the carbonaceousmaterial having diameters in the range of 3 to 40 μm, the circularity isdesirably as close to 1 as possible, and the circularity is preferably0.1 or above, more preferably 0.5 or above, still more preferably 0.8 orabove, further preferably 0.85 or above, and particularly preferably 0.9or above. Because particles having a higher circularity can achieve ahigher fill factor and the resistance between such particles is small,high-current density charge/discharge characteristics are enhanced withincreasing circularity. Thus, particles having a higher circularity inthe above range are more preferable.

The circularity is measured with a flow-type particle image analyzer(FPIA manufactured by Sysmex Corporation). Approximately 0.2 g of thesample is dispersed in a 0.2 mass % aqueous solution (approximately 50mL) of polyoxyethylene (20) sorbitan monolaurate as a surfactant, thedispersion is irradiated with 28 kHz ultrasonic waves at an output of 60W for 1 minute, and particles having diameters in the range of 3 to 40μm are analyzed while setting the detection range to 0.6 to 400 μm.

The circularity may be increased by any method without limitation. Aspheronization treatment is preferable because the obtainable sphereparticles can form an electrode having a uniform shape of the gapsbetween the particles. Examples of the spheronization treatments includemechanical spheronization methods by the application of shear force orcompressive force, and mechanical/physical treatment methods in whichfine particles are unified with a binder or by the adhesion of theparticles themselves.

(Tap Density)

The tap density of the carbonaceous material is usually 0.1 g·cm⁻³ orabove, preferably 0.5 g·cm⁻³ or above, more preferably 0.7 g·cm⁻³ orabove, and particularly preferably 1 g·cm⁻³ or above, and is preferably2 g·cm⁻³ or less, more preferably 1.8 g·cm⁻³ or less, and particularlypreferably 1.6 g·cm⁻³ or less. When the tap density is in this range,the battery capacity can be ensured and the increase in the resistancebetween the particles can be suppressed.

The tap density is measured by allowing the particles to fall into a 20cm³ tapping cell through a sieve having a mesh opening of 300 μm untilthe sample reaches the upper end of the cell, and tapping the cell 1000times with a stroke length of 10 mm with use of a powder density meter(for example, Tap Denser manufactured by Seishin Enterprise Co., Ltd.).The tap density is calculated based on the volume and the mass of thesample.

(Orientation Ratio)

The orientation ratio of the carbonaceous material is usually 0.005 orabove, preferably 0.01 or above, and more preferably 0.015 or above, andis usually 0.67 or less. When the orientation ratio is in this range,excellent high-density charge/discharge characteristics can be ensured.The upper limit of the above range is the theoretical upper limit of theorientation ratio of the carbonaceous materials.

The orientation ratio is measured by X-ray diffractometry with respectto a compact of the sample. The sample weighing 0.47 g is packed into amolding machine 17 mm in diameter and is compressed at 58.8 MN·m⁻². Withuse of clay, the resultant compact is set so that the plane of thesample is on the same level as the plane of the measurement sampleholder, and an X-ray diffraction spectrum is measured. Based on the peakintensities of (110) diffraction and (004) diffraction of carbon, the(110) diffraction peak intensity/(004) diffraction peak intensity ratiois calculated.

The X-ray diffractometry conditions are as follows. “20” indicates thediffraction angle.

-   -   Target: Cu (Kα ray) graphite monochromator    -   Slits:        -   Divergence slit=0.5 degrees        -   Receiving slit=0.15 mm        -   Scattering slit=0.5 degrees    -   Measurement range and step angle/measurement time:

(110) Plane: 75 degrees≤2θ≤80 degrees, 1 degree/60 seconds

(004) Plane: 52 degrees≤2θ≤57 degrees, 1 degree/60 seconds

(Aspect Ratio (Powder))

The aspect ratio of the carbonaceous material is usually 1 or more, andis usually 10 or less, preferably 8 or less, and more preferably 5 orless. This aspect ratio ensures that the material can be applied withenhanced uniformity while suppressing the occurrence of streaks on theelectrodes in forming an electrode, thus making it possible to ensureexcellent high-current density charge/discharge characteristics. Thelower limit of the above range is the theoretical lower limit of theaspect ratio of the carbonaceous materials.

The aspect ratio is measured with respect to particles of thecarbonaceous material enlarged on a scanning electron microscope. Fiftygraphite particles are selected randomly from the particles fixed to anend of a metal having a thickness of 50 μm or less. The stage on whichthe sample is fixed is rotated and tilted to observe each of theparticles three-dimensionally. The largest diameter A of the particle ofthe carbonaceous material and the smallest diameter B that isperpendicular to the largest diameter A are measured. The average of theA/B ratios is determined.

Configurations and Methods for Fabrication of Negative Electrodes

The electrodes may be produced by any known methods as long as theadvantageous effects of the invention are not significantly impaired.For example, the electrode may be formed by combining the negativeelectrode active material with a binder, a solvent and optionally withadditives such as a thickener, a conductive material and a filler togive a slurry, and applying the slurry to a current collector followedby drying and pressing.

In the case of using the alloy material, a thin layer containing thenegative electrode active material (a negative electrode active materiallayer) may be formed by a technique such as deposition, sputtering orplating.

(Current Collectors)

The current collectors on which the negative electrode active materialis held may be any known collectors. Examples of the negative electrodecurrent collectors include metal materials such as aluminum, copper,nickel, stainless steel and nickel-plated steel. In terms of easyprocessing and cost, copper is particularly preferable.

Examples of the shapes of the metallic current collectors include metalfoils, metal cylinders, metal coils, metal plates, metal thin films,expanded metals, punched metals and porous metals. Metal thin films arepreferable, and copper foils are more preferable. Rolled copper foilsobtained by a rolling method, and electrolytic copper foils obtained byan electrolytic method are still more preferable for use as the currentcollectors.

From the points of view of ensuring the battery capacity and handlingproperties, the thickness of the current collectors is usually 1 μm ormore, and preferably 5 μm or more, and is usually 100 μm or less, andpreferably 50 μm or less.

(Thickness Ratio of Negative Electrode Active Material Layer to CurrentCollector)

The thickness ratio of the negative electrode active material layer tothe current collector is not particularly limited. It is, however,preferable that the value of “(Thickness of negative electrode activematerial layer per side immediately before pouring of nonaqueouselectrolytic solution)/(Thickness of current collector)” be 150 or less,more preferably 20 or less, and particularly preferably 10 or less, andbe 0.1 or more, more preferably 0.4 or more, and particularly preferably1 or more. When the thickness ratio of the negative electrode activematerial layer to the current collector is in the above range, thebattery capacity can be ensured while the generation of heat from thecurrent collector during charging and discharging at a high currentdensity can be suppressed.

(Binders)

The binders for binding the particles of the negative electrode activematerial are not particularly limited as long as the binders are stableto the nonaqueous electrolytic solution and the solvents used in theproduction of the electrodes.

Specific examples include resin polymers such as polyethylene,polypropylene, polyethylene terephthalate, polymethyl methacrylate,aromatic polyamide, polyimide, cellulose and nitrocellulose; rubberpolymers such as SBR (styrene-butadiene rubber), isoprene rubber,butadiene rubber, fluororubber, NBR (acrylonitrile-butadiene rubber) andethylene-propylene rubber, styrene-butadiene-styrene block copolymer andhydrogenated products thereof; thermoplastic elastomeric polymers suchas EPDM (ethylene-propylene-diene terpolymer),styrene-ethylene-butadiene-styrene copolymer, styrene-isoprene-styreneblock copolymer and hydrogenated products of these polymers; soft resinpolymers such as syndiotactic 1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymer and propylene-α-olefin copolymer;fluorine-containing polymers such as polyvinylidene fluoride,polytetrafluoroethylene, fluorinated polyvinylidene fluoride andpolytetrafluoroethylene-ethylene copolymer, and polymer compositionshaving conductivity for alkali metal ions (in particular, lithium ions).These binders may be used singly, or two or more may be used incombination in an appropriate ratio.

The ratio of the binder to the negative electrode active material ispreferably 0.1 mass % or above, more preferably 0.5 mass % or above, andparticularly preferably 0.6 mass % or above, and is preferably 20 mass %or less, more preferably 15 mass % or less, still more preferably 10mass % or less, and particularly preferably 8 mass % or less. This ratioof the binder to the negative electrode active material ensures asufficient battery capacity and a sufficient strength of the negativeelectrodes.

When, in particular, the binder includes a rubber polymer such as SBR asthe main component, the ratio of the binder to the negative electrodeactive material is usually 0.1 mass % or above, preferably 0.5 mass % orabove, and more preferably 0.6 mass % or above, and is usually 5 mass %or less, preferably 3 mass % or less, and more preferably 2 mass % orless. When the binder includes a fluorine-containing polymer such aspolyvinylidene fluoride as the main component, the ratio of the binderto the negative electrode active material is usually 1 mass % or above,preferably 2 mass % or above, and more preferably 3 mass % or above, andis usually 15 mass % or less, preferably 10 mass % or less, and morepreferably 8 mass % or less.

(Slurry-Forming Solvents)

The solvents for forming the slurry may be any kinds of solvents withoutlimitation as long as the negative electrode active material, thebinder, and a thickener and a conductive material that are optionallyused can be dissolved or dispersed in the solvents. The solvents may beaqueous solvents or organic solvents.

Examples of the aqueous solvents include water and alcohols. Examples ofthe organic solvents include N-methylpyrrolidone (NMP),dimethylformamide, dimethylacetamide, methyl ethyl ketone,cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine,N,N-dimethylaminopropyiamine, tetrahydrofuran (THF), toluene, acetone,diethyl ether, hexamethylphosphoramide, dimethylsulfoxide, benzene,xylene, quinoline, pyridine, methylnaphthalene and hexane.

When, in particular, an aqueous solvent is used, it is preferable thatthe slurry be prepared while adding a dispersant in combination with athickener and while using a latex of SBR. The solvents may be usedsingly, or two or more may be used in combination in an appropriateratio.

(Thickeners)

The thickener is usually used to control the viscosity of the slurry.The thickeners are not particularly limited. Specific examples includecarboxymethylcellulose, methylcellulose, hydroxymethylcellulose,ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylatedstarch, casein and salts of these compounds. The thickeners may be usedsingly, or two or more may be used in combination in an appropriateratio.

When the thickener is used, the ratio of the thickener to the negativeelectrode active material is usually 0.1 mass % or above, preferably 0.5mass % or above, and more preferably 0.6 mass % or above, and is usually5 mass % or less, preferably 3 mass % or less, and more preferably 2mass % or less. When the ratio of the thickener to the negativeelectrode active material is in this range, it is possible to suppressthe decrease in battery capacity and the increase in resistance, andalso to ensure good application properties.

(Electrode Density)

The electrode structure of the electrodes formed of the negativeelectrode active material is not particularly limited. It is, however,preferable that the density of the negative electrode active materialpresent on the current collector be 1 g·cm⁻³ or more, more preferably1.2 g·cm⁻³ or more, and particularly preferably 1.3 g·cm⁻³ or more, andbe 2.2 g·cm⁻³ or less, more preferably 2.1 g·cm⁻³ or less, still morepreferably 2.0 g·cm⁻³ or less, and particularly preferably 1.9 g·cm⁻³ orless. When the density of the negative electrode active material presenton the current collector is in this range, the particles of the negativeelectrode active material are prevented from breakage. Further, theabove density makes it possible to prevent an increase in initialirreversible capacity and to suppress deteriorations in high-currentdensity charge/discharge characteristics due to poor accessibility ofthe nonaqueous electrolytic solution to the vicinity of the currentcollector/negative electrode active material interface. Furthermore, theabove density ensures that the decrease in battery capacity and theincrease in resistance are prevented.

(Thickness of Negative Electrode Plates)

The thickness of the negative electrode plates is not particularlylimited and is designed in accordance with the positive electrode platesthat are used. The thickness of the mixture layer excluding thethickness of the metal foil as the core is usually 15 μm or more,preferably 20 μm or more, and more preferably 30 μm or more, and isusually 300 μm or less, preferably 280 μm or less, and more preferably250 μm or less.

(Coatings on Surface of Negative Electrode Plates)

Substances having a composition different from that of the negativeelectrode plates may be attached to the surface of the negativeelectrode plates. Examples of such adherent substances include oxidessuch as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide and bismuthoxide, sulfate salts such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate and aluminum sulfate, andcarbonate salts such as lithium carbonate, calcium carbonate andmagnesium carbonate.

2-2. Positive Electrodes

<Positive Electrode Active Materials>

Hereinbelow, positive electrode active materials used in the positiveelectrodes will be described.

(Compositions)

The positive electrode active materials are not particularly limited aslong as the materials can electrochemically store and release metal ions(for example, lithium ions). For example, materials containing lithiumand at least one transition metal are preferable. Specific examplesinclude lithium-transition metal composite oxides and lithium-transitionmetal phosphate compounds.

Preferred examples of the transition metals in the lithium-transitionmetal composite oxides include V, Ti, Cr, Mn, Fe, Co, Ni and Cu.Specific examples include lithium-cobalt composite oxides such asLiCoO₂, lithium-nickel composite oxides such as LiNiO₂,lithium-manganese composite oxides such as LiMnO₂, LiMn₂O₄ and Li₂MnO₄,lithium-nickel-manganese-cobalt composite oxides such asLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ and LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, andcomposite oxides corresponding to the above lithium-transition metalcomposite oxides except that the main transition metal atoms arepartially substituted by other elements such as Na, K, B, F, Al, Ti, V,Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Nb, Mo, Sn and W.Examples of such substituted composite oxides includeLiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.85)Co_(0.10)Al_(0.05)O₂,LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂, LiNi_(0.45)Co_(0.10)Al_(0.45)O₂,LiMn_(1.8)Al_(0.2)O₄ and LiMn_(1.5)Ni_(0.5)O₄.

Preferred examples of the transition metals in the lithium-transitionmetal phosphate compounds include V, Ti, Cr, Mn, Fe, Co, Ni and Cu.Specific examples of the compounds include iron phosphates such asLiFePO₄, Li₃Fe₂(PO₄)₃ and LiFeP₂O₇, cobalt phosphates such as LiCoPO₄,and compounds corresponding to the above lithium-transition metalphosphate compounds except that the main transition metal atoms arepartially substituted by other elements such as Al, Ti, V, Cr, Mn, Fe,Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb and Si.

The addition of lithium phosphate to the positive electrode activematerial advantageously enhances continuous charging characteristics.The manner in which lithium phosphate is used is not limited.Preferably, lithium phosphate is used as a mixture with the positiveelectrode active material described above. With respect to the total ofthe positive electrode active material and lithium phosphate, the lowerlimit of the amount of lithium phosphate is preferably 0.1 mass % orabove, more preferably 0.3 mass % or above, and still more preferably0.5 mass % or above, and the upper limit is preferably 10 mass % orless, more preferably 8 mass % or less, and still more preferably 5 mass% or less.

(Surface Coatings)

Substances having a composition different from that of the positiveelectrode active material may be attached to the surface of the positiveelectrode active material. Examples of such adherent substances includeoxides such as aluminum oxide, silicon oxide, titanium oxide, zirconiumoxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide andbismuth oxide, sulfate salts such as lithium sulfate, sodium sulfate,potassium sulfate, magnesium sulfate, calcium sulfate and aluminumsulfate, carbonate salts such as lithium carbonate, calcium carbonateand magnesium carbonate, and carbon.

For example, these adherent substances may be attached to the surface ofthe positive electrode active material by a method in which the positiveelectrode active material is impregnated with a solution or suspensionof the substance in a solvent, and the wet material is dried; a methodin which the positive electrode active material is impregnated with asolution or suspension of an adherent substance precursor in a solvent,and the wet material is treated by heating or the like to perform thereaction of the precursor; and a method in which the substance is addedto a positive electrode active material precursor and the mixture iscalcined together. In the case of carbon, for example, a carbonaceoussubstance may be mechanically attached in the form of activated carbonor the like after the production of the active material.

The lower limit of the mass of the adherent substance relative to thepositive electrode active material is preferably 0.1 ppm or more, morepreferably 1 ppm or more, and still more preferably 10 ppm or more, andthe upper limit thereof is preferably 20% or less, more preferably 10%or less, and still more preferably 5% or less. The adherent substancecan suppress the oxidation reaction of the electrolytic solution on thesurface of the positive electrode active material, thereby extending thebattery life. If the amount of the substance attached is too small,these effects may not be obtained sufficiently. If present in anexcessively large amount, the adherent inhibits the entry and exit oflithium ions to cause an increase in resistance at times.

In the invention, the positive electrode active materials carrying anadherent substance with a different composition on the surface are alsowritten as the “positive electrode active materials”.

(Shapes)

The shapes of the particles of the positive electrode active materialmay be conventional shapes such as bulky masses, polyhedrons, spheres,ellipses, plates, needles and columns. Further, primary particles may beaggregated to form secondary particles.

(Tap Density)

The tap density of the positive electrode active material is preferably0.5 g/cm³ or above, more preferably 0.8 g/cm³ or above, and still morepreferably 1.0 g/cm³ or above. When the tap density of the positiveelectrode active material is in this range, it is possible to reduce theamounts of a dispersion medium, a conductive material and a binderrequired to form positive electrode active material layers, andconsequently it is possible to ensure a high fill factor of the positiveelectrode active material and a high battery capacity. Dense positiveelectrode active material layers may be formed by using a compositeoxide powder having a high tap density. In general, a higher tap densityis more preferable, and the upper limit is not particularly limited. Itis, however, preferable that the tap density be 4.0 g/cm³ or less, morepreferably 3.7 g/cm³ or less, and still more preferably 3.5 g/cm³ orless. This tap density ensures that the decrease in load characteristicsis prevented.

In the invention, the tap density is measured in such a manner that 5 to10 g of the positive electrode active material powder is placed into a10 ml glass graduated cylinder, which is then tapped 200 times with astroke of about 20 mm, and the packing density (the tap density) of thepowder is measured in terms of g/cc.

(Median Diameter d50)

The median diameter d50 of the particles of the positive electrodeactive material (the secondary particle diameter when the primaryparticles are aggregated into secondary particles) is preferably 0.3 μmor more, more preferably 0.5 μm or more, still more preferably 0.8 μm ormore, and most preferably 1.0 μm or more, and the upper limit ispreferably 30 μm or less, more preferably 27 μm or less, still morepreferably 25 μm or less, and most preferably 22 μm or less. This mediandiameter ensures that a high tap density is obtained and the decrease inbattery performance is prevented, and also ensures that a slurry of theactive material and other components such as a conductive material and abinder in a solvent can be applied to form thin films as positiveelectrodes of batteries while preventing the occurrence of problems suchas streaks. The fill factor in the fabrication of positive electrodesmay be further enhanced by using a mixture of two or more kinds of thepositive electrode active materials having different median diametersd50.

In the invention, the median diameter d50 is measured with a known laserdiffraction/scattering grain size distribution analyzer. When LA-920manufactured by Horiba, Ltd. is used as the grain size distributionanalyzer, the particles are dispersed in a 0.1 mass % aqueous sodiumhexametaphosphate solution as the dispersion medium by the applicationof ultrasonic waves for 5 minutes, and the diameters are measured whilesetting the refractive index at 1.24.

(Average Primary Particle Diameter)

When the primary particles are aggregated into secondary particles, theaverage primary particle diameter of the positive electrode activematerial is preferably 0.05 μm or more, more preferably 0.1 μm or more,and still more preferably 0.2 μm or more, and the upper limit ispreferably 5 μm or less, more preferably 4 μm or less, still morepreferably 3 μm or less, and most preferably 2 μm or less. This averagediameter ensures that a good fill factor and a sufficient specificsurface area of the powder are obtained and the decrease in batteryperformance is prevented, and also ensures that the particles haveappropriate crystallinity to ensure reversibility of charging anddischarging.

In the invention, the primary particle diameters are measured byobservation using a scanning electron microscope (SEM). Specifically,the largest length of a segment defined by a horizontal straight lineintersecting the primary particle is measured with respect to fiftyprimary particles randomly selected in a xl 0000 photograph, the resultsbeing averaged.

(BET Specific Surface Area)

The BET specific surface area of the positive electrode active materialis preferably 0.1 m²/g or more, more preferably 0.2 m²/g or more, andstill more preferably 0.3 m²/g or more, and the upper limit is 50 m²/gor less, preferably 40 m²/g or less, and more preferably 30 m²/g orless. When the BET specific surface area is in this range, a goodbattery performance is ensured while maintaining good applicationproperties of the positive electrode active material.

In the invention, the BET specific surface area is defined as a valuemeasured using a surface area meter (for example, an automatic surfacearea measuring apparatus manufactured by Okura Riken) in such a mannerthat the sample is preliminarily dried at 150° C. under a stream ofnitrogen for 30 minutes and thereafter the analysis is performed by thenitrogen adsorption BET single point method by flowing a nitrogen-heliummixed gas prepared so that the pressure of nitrogen relative to theatmospheric pressure is exactly 0.3.

(Methods for Producing Positive Electrode Active Materials)

The positive electrode active material may be produced by a commonmethod for the production of inorganic compounds. In particular, whilevarious methods may be used to produce spherical or elliptical activematerials, an exemplary method is such that a transition metal rawmaterial is dissolved or is crushed and dispersed in a solvent such aswater, the pH is adjusted while performing stirring so as to form aspherical precursor, which is then recovered and dried as required,thereafter a Li source such as LiOH, Li₂CO₃ or LiNO₃ is added, and themixture is calcined at a high temperature to give the active material.

In the production of positive electrodes, the positive electrode activematerials described hereinabove may be used singly or in combinationwith one or more positive electrode active materials having a differentcomposition in an appropriate ratio. In this case, some preferredcombinations are combinations of LiCoO₂ with LiMn₂O₄ or a similarstructure such as LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ resulting from partialsubstitution of Mn by other metals such as transition metals, or withLiCoO₂ or a similar structure resulting from partial substitution of Coby other metals such as transition metals.

Configurations and Methods for Fabrication of Positive Electrodes

Hereinbelow, configurations of positive electrodes will be described. Inthe invention, the positive electrode may be fabricated by forming apositive electrode active material layer containing the positiveelectrode active material and a binder, onto a current collector. Thepositive electrodes may be produced using the positive electrode activematerial by a conventional method. That is, the positive electrode maybe obtained by dry-mixing the positive electrode active material, abinder and optionally other components such as a conductive material anda thickener, and compression bonding a sheet of the mixture to apositive electrode current collector, or by dissolving or dispersingthese materials in a liquid medium to give a slurry, applying the slurryonto a positive electrode current collector, and drying the wet film toform a positive electrode active material layer on the currentcollector.

In the positive electrode active material layer, the content of thepositive electrode active material is preferably 80 mass % or above,more preferably 82 mass % or above, and particularly preferably 84 mass% or above. The upper limit is preferably 99 mass % or less, and morepreferably 98 mass % or less. With this content, the electrical capacityof the positive electrode active material in the positive electrodeactive material layer may be ensured, and also the strength of thepositive electrodes may be maintained. In order to increase the packingdensity of the positive electrode active material, it is preferable thatthe positive electrode active material layer formed by application anddrying be pressed with a device such as a hand press or a roller press.The lower limit of the density of the positive electrode active materiallayer is preferably 1.5 g/cm³, more preferably 2 g/cm³, and still morepreferably 2.2 g/cm³, and the upper limit is preferably 5 g/cm³, morepreferably 4.5 g/cm³, and still more preferably 4 g/cm³. This densityensures that good charge and discharge characteristics may be obtainedand the increase in electrical resistance may be suppressed.

(Conductive Materials)

The conductive materials may be any known conductive materials. Specificexamples include metal materials such as copper and nickel; and carbonmaterials, for example, black leads (graphites) such as natural graphiteand artificial graphite, carbon blacks such as acetylene black, andamorphous carbons such as needle cokes. These materials may be usedsingly, or two or more may be used in combination in an appropriateratio. The content of the conductive material(s) in the positiveelectrode active material layer is usually 0.01 mass % or above,preferably 0.1 mass % or above, and more preferably 1 mass % or above,and the upper limit is usually 50 mass % or less, preferably 30 mass %or less, and more preferably 15 mass % or less. This content ensuresthat sufficient conductive properties and sufficient battery capacitymay be obtained.

(Binders)

The binders used in the production of the positive electrode activematerial layers are not particularly limited. When the layers are formedby an application method, any binders which may be dissolved ordispersed in the liquid media used in the electrode production may beused. Specific examples include resin polymers such as polyethylene,polypropylene, polyethylene terephthalate, polymethyl methacrylate,polyimide, aromatic polyamide, cellulose and nitrocellulose; rubberpolymers such as SBR (styrene-butadiene rubber), NBR(acrylonitrile-butadiene rubber), fluororubber, isoprene rubber,butadiene rubber and ethylene-propylene rubber; thermoplasticelastomeric polymers such as styrene-butadiene-styrene block copolymerand hydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymer), styrene-ethylene-butadiene-ethylene copolymer, andstyrene-isoprene-styrene block copolymer and hydrogenated productsthereof; soft resin polymers such as syndiotactic 1,2-polybutadiene,polyvinyl acetate, ethylene-vinyl acetate copolymer andpropylene-α-olefin copolymer; fluorine-containing polymers such aspolyvinylidene fluoride (PVdF), polytetrafluoroethylene, fluorinatedpolyvinylidene fluoride and polytetrafluoroethylene-ethylene copolymer,and polymer compositions having conductivity for alkali metal ions (inparticular, lithium ions). These binders may be used singly, or two ormore may be used in combination in an appropriate ratio.

The proportion of the binder in the positive electrode active materiallayer is usually 0.1 mass % or above, preferably 1 mass % or above, andmore preferably 1.5 mass % or above, and the upper limit is usually 80mass % or less, preferably 60 mass % or less, more preferably 40 mass %or less, and most preferably 10 mass % or less. When used in anexcessively low proportion, the binder fails to hold the positiveelectrode active material sufficiently and the mechanical strength ofthe positive electrode is decreased, possibly resulting in a decrease inbattery performance such as cycle characteristics. On the other hand,adding an excessively large amount of the binder results in a decreasein battery capacity or conductive properties at times.

(Slurry-Forming Solvents)

The solvents for forming the slurry may be any kinds of solvents withoutlimitation as long as the positive electrode active material, theconductive material, the binder and an optional thickener can bedissolved or dispersed in the solvents. The solvents may be aqueoussolvents or organic solvents. Examples of the aqueous media includewater, and mixed media of alcohols and water. Examples of the organicmedia include aliphatic hydrocarbons such as hexane; aromatichydrocarbons such as benzene, toluene, xylene and methylnaphthalene;heterocyclic compounds such as quinoline and pyridine; ketones such asacetone, methyl ethyl ketone and cyclohexanone; esters such as methylacetate and methyl acrylate; amines such as diethylenetriamine andN,N-dimethylaminopropylamine; ethers such as diethyl ether, propyleneoxide and tetrahydrofuran (THF); amides such as N-methylpyrrolidone(NMP), dimethylformamide and dimethylacetamide; and aprotic polarsolvents such as hexamethylphosphoramide and dimethylsulfoxide.

When, in particular, an aqueous medium is used, it is preferable thatthe slurry be prepared while adding a thickener and while using a latexof styrene-butadiene rubber (SBR). The thickener is usually used tocontrol the viscosity of the slurry. The thickeners are not particularlylimited. Specific examples include carboxymethylcellulose,methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinylalcohol, oxidized starch, phosphorylated starch, casein and salts ofthese compounds. The thickeners may be used singly, or two or more maybe used in combination in an appropriate ratio. When the thickener isused, the ratio of the thickener to the active material is 0.1 mass % orabove, preferably 0.2 mass % or above, and more preferably 0.3 mass % orabove, and the upper limit is 5 mass % or less, preferably 3 mass % orless, and more preferably 2 mass % or less. This ratio ensures that goodapplication properties may be obtained, and the decrease in batterycapacity and the increase in resistance may be prevented.

(Current Collectors)

The materials of the positive electrode current collectors are notparticularly limited, and any known materials may be used. Specificexamples include metal materials such as aluminum, stainless steel,nickel plating, titanium and tantalum; and carbon materials such ascarbon cloth and carbon paper. Of these, metal materials are preferable,and aluminum is particularly preferable.

Examples of the shapes of the metallic current collectors include metalfoils, metal cylinders, metal coils, metal plates, metal thin films,expanded metals, punched metals and porous metals. Examples of theshapes of the carbon current collectors include carbon plates, carbonthin films and carbon cylinders. Of these, metal thin films arepreferable. The thin films may be in the form of meshed films asappropriate. The thickness of the thin films is not limited. From thepoints of view of the strength and the handling properties of thecurrent collectors, the thickness is usually 1 μm or more, preferably 3μm or more, and more preferably 5 μm or more, and the upper limit isusually 1 mm or less, preferably 100 μm or less, and more preferably 50μm or less.

In order to reduce the electronic contact resistance between the currentcollector and the positive electrode active material layer, it is alsopreferable that the surface of the current collector be coated with aconductive auxiliary. Examples of the conductive auxiliaries includecarbon and noble metals such as gold, platinum and silver.

The thickness ratio of the positive electrode active material layer tothe current collector is not particularly limited. It is, however,preferable that the value of (Thickness of positive electrode activematerial layer per side immediately before pouring of electrolyticsolution)/(Thickness of current collector) be 20 or less, morepreferably 15 or less, and most preferably 10 or less, and the lowerlimit be 0.5 or more, more preferably 0.8 or more, and most preferably 1or more. If the ratio is above this range, the current collector maygenerate Joule heat during charging and discharging at a high currentdensity. The satisfaction of the above range ensures that the generationof heat from the current collector during charging and discharging at ahigh current density may be suppressed and the battery capacity may beensured.

(Electrode Area)

In order to increase the output and the stability at high temperaturesin the use of the electrolytic solution of the invention, it ispreferable that the area of the positive electrode active materiallayers be larger than the outer surface area of a battery exterior case.Specifically, the total of the electrode areas of the positive electrodeis preferably 15 times or more, and more preferably 40 times or morelarger than the surface area of the exterior of the secondary battery.In the case of a bottomed square case, the outer surface area of theexterior case is the total area calculated from the length, the widthand the thickness of the case accommodating the electricity-generatingelement except the projections of the terminals. In the case of abottomed cylindrical case, the outer surface area is the geometricsurface area obtained by approximating that the case accommodating theelectricity-generating element except the projections of the terminalsis a cylinder. The total of the electrode areas of the positiveelectrode is the geometric surface area of the positive electrodemixture layer(s) opposed to the mixture layer(s) including the negativeelectrode active material. In the case where the positive electrodemixture layers are formed on both sides of the current collector foil,the total of the electrode areas is the total of the areas of thesurfaces calculated separately.

(Thickness of Positive Electrode Plates)

The thickness of the positive electrode plates is not particularlylimited. From the points of view of high capacity and high output, thelower limit of the thickness of the mixture layers per side of thecurrent collector obtained by subtracting the thickness of the metalfoil as the core is preferably 10 μm or more, and more preferably 20 μmor more, and the upper limit is preferably 500 μm or less, and morepreferably 450 μm or less.

(Coatings on Surface of Positive Electrode Plates)

Substances having a composition different from that of the positiveelectrode plates may be attached to the surface of the positiveelectrode plates. Examples of such adherent substances include oxidessuch as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide and bismuthoxide, sulfate salts such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate and aluminum sulfate,carbonate salts such as lithium carbonate, calcium carbonate andmagnesium carbonate, and carbon.

2-3. Separators

A separator is usually disposed between the positive electrode and thenegative electrode to prevent short-circuits. In this case, theseparator is usually impregnated with the electrolytic solution of theinvention.

The materials and the shapes of the separators are not particularlylimited, and known materials and shapes may be used appropriately aslong as the advantageous effects of the invention are not significantlyimpaired. In particular, use may be made of those materials which arestable to the electrolytic solution of the invention such as resins,glass fibers and inorganic substances. Those having excellent liquidretention properties such as porous sheets and nonwoven fabrics arepreferably used.

Examples of the materials for the resin and glass fiber separatorsinclude polyolefins such as polyethylene and polypropylene, aromaticpolyamide, polytetrafluoroethylene, polyethersulfone and glass filters.In particular, glass filters and polyolefins are preferable, polyolefinsare more preferable, and polypropylene is particularly preferable. Thesematerials may be used singly, or two or more may be mixed or stackedtogether in an appropriate ratio. Specific examples of the stacks of twoor more materials include three-layer separators in which polypropylene,polyethylene and polypropylene are stacked together in this order.

The thickness of the separators is not limited, but is usually 1 μm ormore, preferably 5 μm or more, and more preferably 8 μm or more, and isusually 50 μm or less, preferably 40 μm or less, and more preferably 30μm or less. This thickness ensures insulating properties and mechanicalstrength, and also ensures battery performance such as ratecharacteristics and energy density.

When a porous separator such as a porous sheet or a nonwoven fabric isused, the porosity of the separator is not limited, but is usually 20%or more, preferably 35% or more, and more preferably 45% or more, and isusually 90% or less, preferably 85% or less, and more preferably 75% orless. This porosity ensures insulating properties and mechanicalstrength and also ensures that the film resistance may be reduced andgood rate characteristics may be obtained.

The average pore diameter of the separators is not limited, but isusually 0.5 μm or less, and preferably 0.2 μm or less, and is usually0.05 μm or more. Any average pore diameter exceeding the above rangeincreases the probability of short-circuits. The above average porediameter ensures that the film resistance is low and good ratecharacteristics are obtained while preventing the occurrence ofshort-circuits. Examples of the inorganic substances as the materialsinclude oxides such as alumina and silicon dioxide, nitrides such asaluminum nitride and silicon nitride, and sulfate salts such as bariumsulfate and calcium sulfate. Particulate or fibrous inorganic substancesare used.

The forms of such separators may be thin films such as nonwoven fabrics,woven fabrics and microporous films. Thin-film separators having a porediameter of 0.01 to 1 μm and a thickness of 5 to 50 μm are suitablyused. While the separators may be independent thin films as describedabove, composite porous layers including particles of the inorganicsubstance and a resin binder may be formed as the separators on thesurface of the positive electrodes and/or the negative electrodes. Forexample, porous layers may be formed on both sides of the positiveelectrode using alumina particles having a D90 particle diameter of lessthan 1 μm and a fluororesin as a binder.

2-4. Battery Designs

Electrode Assemblies

The electrode assembly may be a stack of the positive electrode plateand the negative electrode plate through the separator, or may be astructure in which the positive electrode plate and the negativeelectrode plate are wound through the separator into a coil. Theproportion of the volume of the electrode assembly to the inner volumeof the battery (hereinafter, written as the electrode assembly occupancyrate) is usually 40% or above, and preferably 50% or above, and isusually 90% or less, and preferably 80% or less. When the electrodeassembly occupancy rate is in this range, it is possible to ensure thebattery capacity and to prevent inconveniences associated with theincrease in internal pressure, for example, deteriorations incharacteristics such as charge-discharge repetition characteristics andhigh-temperature storage characteristics, and the actuation of gasrelease valves.

Current-Collecting Structures

The current-collecting structures are not particularly limited. Suchstructures that the resistance at wirings and joints is low arepreferable. In the case of a stack electrode assembly, thecurrent-collecting structure is suitably formed by welding bundles ofthe metallic cores of the respective electrode layers to terminals. Inview of the fact that a large electrode has a high internal resistance,it is preferable that a plurality of terminals be disposed in such anelectrode to decrease the resistance. In the case of a wound electrodeassembly, a plurality of leads may be provided on each of the positiveand the negative electrodes and the bundles of the leads may beconnected to respective terminals, thereby reducing the internalresistance.

Exterior Cases

The materials of the exterior cases are not particularly limited as longas they are stable to the nonaqueous electrolytic solution used.Specific examples include metals such as nickel-plated steel sheets,stainless steel, aluminum, aluminum alloys and magnesium alloys, andresin/aluminum foil stack films (laminate films). From the point of viewof weight reduction, metals such as aluminum and aluminum alloys, andlaminate films are suitably used.

Examples of the metallic exterior cases include tightly sealedstructures formed by welding the metals by laser welding, resistancewelding or ultrasonic welding, and structures formed by caulking orcrimping the metals via resin gaskets. Examples of the laminate-filmexterior cases include tightly sealed structures formed by fusionbonding the resin layers. To enhance sealing properties, the resinlayers may be fusion bonded through a resin different from the resinused in the laminate film. When, in particular, a sealed structure isproduced by fusion bonding the resin layers while the current-collectingterminals are interposed between the resin layers, a polargroup-containing resin or a resin modified by the introduction of apolar group is suitably used to mediate the bond between the metal andthe resin. The shapes of the exterior bodies are not limited and may be,for example, any of cylindrical shapes, prismatic shapes, laminateshapes, coin shapes and large shapes.

Protective Elements

Protective elements may be used such as PTC (positive temperaturecoefficient) detectors, thermal fuses and thermistors that increaseresistance in the event of abnormal heat generation or overcurrent, andvalves (current cutoff valves) that interrupt the flow of currentpassing through the circuits upon a rapid increase in internal pressureor internal temperature in the batteries due to abnormal heatgeneration. It is preferable to select protective elements that do notoperate at a high current under normal conditions. It is more preferableto design the batteries so that any abnormal heat generation or thermalrunaway does not occur even in the absence of protective elements.

EXAMPLES

Hereinbelow, the present invention will be described in greater detailbased on Examples and Comparative Examples. The scope of the inventionis not limited to such Examples.

The structures of aromatic carboxylate esters of Formula (1) used inExamples are illustrated below.

The structures of aromatic carboxylate esters of Formula (2) used inExamples are illustrated below.

The structures of other compounds used are illustrated below.

Example 1-1, and Comparative Examples 1-1 and 1-2 Example 1-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (also written as “EC”), ethylmethyl carbonate (also written as “EMC”) and dimethyl carbonate (alsowritten as “DMC”) (volume ratio 3:4:3) so that its concentration wouldbe 1.0 mol/L, thus forming a basic electrolytic solution. Further, 1.0mass % of the compound (1-1) as an additive was added to the basicelectrolytic solution. In this manner, a nonaqueous electrolyticsolution of Example 1-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 90 mass % of lithium cobalt nickelmanganese oxide (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) as a positive electrodeactive material, 7 mass % of acetylene black as a conductive materialand 3 mass % of polyvinylidene fluoride (PVdF) as a binder were mixedtogether with use of a disperser to give a slurry. The slurry wasuniformly applied onto a surface of a 15 μm thick aluminum foil, and wasdried and pressed. A positive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing an amorphous coated graphite powder as anegative electrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of amorphous coated graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 97.5:1.5:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrode described above, and apolypropylene separator were stacked in the order of the negativeelectrode, the separator and the positive electrode, thus forming abattery element. The battery element was inserted into a bag which wasmade of a laminate film of aluminum (thickness 40 μm) coated with resinlayers on both sides, while ensuring that the terminals of the positiveelectrode and the negative electrode extended beyond the bag.Thereafter, the nonaqueous electrolytic solution was poured into thebag, and the bag was vacuum sealed. In this manner, a sheet-shapednonaqueous electrolyte secondary battery was fabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.2 C to 4.1 V and then at the constant voltage (suchcharging is also written as “CC-CV charging”) (0.05 C cutoff), and wasdischarged to 3.0 V at a constant current of ⅓ C. Thereafter, thebattery was CC-CV charged at a current corresponding to ⅓ C to 4.1 V(0.05 C cutoff) and was allowed to stand at 60° C. for 12 hours. Afterbeing cooled sufficiently, the battery was discharged at a constantcurrent of ⅓ C to 3.0 V. Next, the battery was CC-CV charged at ⅓ C to4.2 V (0.05 C cutoff) and was discharged again at ⅓ C to 3.0 V. In thismanner, initial battery characteristics were stabilized. Thereafter, thebattery was CC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff) and wasdischarged at ⅓ C to 3.0 V, thereby determining the initial ⅓ Ccapacity. Thereafter, the battery was immersed in an ethanol bath andthe volume was measured. The initial gas production was obtained bydetermining the change in volume from the initial volume of the battery.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Evaluation of Overcharge Characteristics]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of ⅓ C to 4.2 V (0.05 C cutoff). Thereafter, thebattery was overcharged at 45° C. and at a constant current of 1 C for48 minutes. After being cooled sufficiently, the open circuit voltage(OCV) of the battery was measured. The OCV after overcharging was thusobtained.

The OCV of a battery after overcharge testing mainly reflects thepotential of a positive electrode. Specifically, a lower OCV afterovercharging indicates a smaller charge depth in a positive electrode.Usually, the increase in the charge depth in a positive electroderesults in the dissolution of metal and the release of oxygen from thepositive electrode, thus initiating a thermal runaway of the battery.Thus, the safety of overcharged batteries may be ensured by thereduction of the OCV after overcharging.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics and theovercharge characteristics. The evaluation results are shown in Table 1relative to the results of Comparative Example 1-1 taken as 100.0%. TheOCV after overcharging is indicated as the difference from the value inComparative Example 1-1. The same applies hereinafter.

Comparative Example 1-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 1-1, except that the electrolytic solutionof Example 1-1 did not contain the compound (1-1).

Comparative Example 1-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 1-1, except that the compound (1-1) usedin the electrolytic solution of Example 1-1 was replaced by 1.0 mass %of the compound (3-2).

TABLE 1 Initial ⅓ C Initial gas OCV after Additive capacity/%production/% overcharging/V Ex. 1-1 Compound (1-1) 100.3 90.1 −69.0 1.0mass % Comp. — 100.0 100.0 0.0 Ex. 1-1 Comp. Compound (3-2) 100.1 102.3−34.5 Ex. 1-2 1.0 mass %

From Table 1, the use of the nonaqueous electrolytic solution of Example1-1 according to the invention resulted in a higher initial ⅓ C capacityand a smaller initial gas production than when no esters of Formula (1)had been added (Comparative Example 1-1). Further, the battery had a lowOCV after overcharging as compared to Comparative Example 1-1, achievinghigher safety. That is, the use of the inventive electrolytic solutionsmakes it possible to obtain batteries having excellent initial batterycharacteristics and excellent safety after overcharge resistancetesting.

When the aromatic compound outside the category of the estersrepresented by Formula (1) was used (Comparative Example 1-2), theinitial rate 1/3 capacity was enhanced as compared to ComparativeExample 1-1 but the improvement was smaller than that obtained inExample 1-1. Further, the initial gas production was increased ascompared to Comparative Example 1-1. Furthermore, the OCV afterovercharging was lower than that in Comparative Example 1-1 but wasinferior to Example 1-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 Example 2-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 3:4:3) so that itsconcentration would be 1.0 mol/L, thus forming a basic electrolyticsolution. Further, 4.5 mass % of the compound (1-1) as an additive wasadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 2-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 90 mass % of lithium cobalt nickelmanganese oxide (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) as a positive electrodeactive material, 7 mass % of acetylene black as a conductive materialand 3 mass % of polyvinylidene fluoride (PVdF) as a binder were mixedtogether with use of a disperser to give a slurry. The slurry wasuniformly applied onto a surface of a 15 μm thick aluminum foil, and wasdried and pressed. A positive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing an amorphous coated graphite powder as anegative electrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of amorphous coated graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 97.5:1.5:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrode described above, and apolypropylene separator were stacked in the order of the negativeelectrode, the separator and the positive electrode, thus forming abattery element. The battery element was inserted into a bag which wasmade of a laminate film of aluminum (thickness 40 μm) coated with resinlayers on both sides, while ensuring that the terminals of the positiveelectrode and the negative electrode extended beyond the bag.Thereafter, the nonaqueous electrolytic solution was poured into thebag, and the bag was vacuum sealed. In this manner, a sheet-shapednonaqueous electrolyte secondary battery was fabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.2 C to 4.1 V and then at the constant voltage (suchcharging is also written as “CC-CV charging”) (0.05 C cutoff), and wasdischarged to 3.0 V at a constant current of ⅓ C. The ratio of thedischarge capacity to the charge capacity obtained during this processwas determined as the first efficiency (%). Thereafter, the battery wasCC-CV charged at a current corresponding to ⅓ C to 4.1 V (0.05 C cutoff)and was allowed to stand at 60° C. for 12 hours. After being cooledsufficiently, the battery was discharged at a constant current of ⅓ C to3.0 V. Next, the battery was CC-CV charged at ⅓ C to 4.2 V (0.05 Ccutoff) and was discharged again at ⅓ C to 3.0 V. In this manner,initial battery characteristics were stabilized.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Evaluation of Overcharge Characteristics]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of ⅓ C to 4.2 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath, and the volume of the battery beforeovercharging was measured based on the buoyancy (Archimedes' principle).Thereafter, the battery was overcharged at 45° C. and at a constantcurrent of 1 C for 48 minutes. After being cooled sufficiently, thebattery was immersed in an ethanol bath and its volume was measured. Thechange in battery volume from before the overcharging was obtained asthe overcharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging, the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics and theovercharge characteristics. The evaluation results are shown in Table 2relative to the results of Comparative Example 2-1 taken as 100.0%. Thesame applies hereinafter.

Example 2-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 2-1, except that the compound (1-1) usedin the electrolytic solution of Example 2-1 was replaced by 4.1 mass %of the compound (1-2). The substance amount of the compound (1-2) usedin Example 2-2 was the same as that of the compound (1-1) used inExample 2-1.

Example 2-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 2-1, except that the compound (1-1) usedin the electrolytic solution of Example 2-1 was replaced by 4.8 mass %of the compound (1-3). The substance amount of the compound (1-3) usedin Example 2-3 was the same as that of the compound (1-1) used inExample 2-1.

Comparative Example 2-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 2-1, except that the electrolytic solutionof Example 2-1 did not contain the compound (1-1).

Comparative Example 2-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 2-1, except that the compound (1-1) usedin the electrolytic solution of Example 2-1 was replaced by 4.1 mass %of the compound (3-3). The substance amount of the compound (3-3) usedin Comparative Example 2-2 was the same as that of the compound (1-1)used in Example 2-1.

Comparative Example 2-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 2-1, except that the compound (1-1) usedin the electrolytic solution of Example 2-1 was replaced by 3.4 mass %of the compound (3-4). The substance amount of the compound (3-4) usedin Comparative Example 2-3 was the same as that of the compound (1-1)used in Example 2-1.

TABLE 2 First Overcharge gas Additive efficiency/% production/% Ex. 2-1Compound (1-1) 4.5 mass % 100.06 427.4 Ex. 2-2 Compound (1-2) 4.1 mass %100.74 305.9 Ex. 2-3 Compound (1-3) 4.8 mass % 100.05 350.4 Comp. —100.00 100.0 Ex. 2-1 Comp. Compound (3-3) 4.1 mass % 98.84 205.9 Ex. 2-2Comp. Compound (3-4) 3.4 mass % 76.24 888.9 Ex. 2-3

From Table 2, the use of the nonaqueous electrolytic solutions ofExamples 2-1 to 2-3 according to the invention resulted in higher firstefficiency than when no esters of Formula (1) had been added(Comparative Example 2-1). Further, the batteries had a large overchargegas production as compared to Comparative Example 2-1, achieving highersafety. That is, the use of the inventive electrolytic solutions makesit possible to obtain batteries having excellent initial batterycharacteristics and excellent safety after overcharge durabilitytesting.

The use of the aromatic compound other than those of Formula (1) alone(Comparative Examples 2-2 and 2-3) resulted in an increase in overchargegas production as compared to Comparative Example 2-1 but also resultedin a significant decrease in initial efficiency from the level ofComparative Example 2-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

Examples 3-1 and 3-2, and Comparative Example 3-1 Example 3-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 15:5:80) so that itsconcentration would be 1.4 mol/L, thus forming a basic electrolyticsolution. Further, 1.5 mass % of the compound (1-4) as an additive wasadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 3-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of a nickel-containingtransition metal oxide as a positive electrode active material, 1.5 mass% of acetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolyethylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 4 hours and was discharged at a constantcurrent of 0.2 C to 2.5 V. Further, the battery was charged at aconstant current corresponding to 0.1 C to 4.1 V and was discharged at aconstant current of 0.2 C to 2.5 V. Next, the battery was CC-CV chargedat 0.2 C to 4.1 V (0.05 C cutoff) and was discharged to 2.5 V at aconstant current of 0.2 C. Next, the battery was CC-CV charged at 0.2 Cto 4.1 V (0.05 C cutoff) and was allowed to stand at 45° C. for 72hours. Thereafter, the battery was discharged again at 25° C. and at 0.2C to 2.5 V. In this manner, initial battery characteristics werestabilized. The initial capacity loss was obtained by determining thedifference between the total of the charge capacities in the fourcharging processes and the total of the discharge capacities in the fourdischarging processes.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Evaluation of Overcharge Characteristics]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.2 V (0.05 C cutoff). The battery was then immersed in an ethanolbath, and the volume of the battery before overcharging was measuredbased on the buoyancy (Archimedes' principle). Thereafter, the batterywas charged at 45° C. and at a constant current of 1 C for 15 minutes.After being cooled sufficiently, the battery was immersed in an ethanolbath and its volume was measured. The change in battery volume frombefore the overcharging was obtained as the overcharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging, the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics and theovercharge characteristics. The evaluation results are shown in Table 3relative to the results of Comparative Example 3-1 taken as 100.0%. Thesame applies hereinafter.

Example 3-2

A nonaqueous electrolyte secondary battery was fabricated and tested toevaluate initial battery characteristics and overcharge characteristicsin the same manner as in Example 3-1, except that 1.5 mass % of thecompound (1-4) used in the electrolytic solution of Example 3-1 wasreplaced by 2.5 mass % of the compound (1-4). The evaluation results areshown in Table 3 relative to the results of Comparative Example 3-1taken as 100.0%. The same applies hereinafter.

Comparative Example 3-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 3-1, except that the electrolytic solutionof Example 3-1 did not contain the compound (1-4).

TABLE 3 Initial capacity Overcharge gas Additive loss/% production/% Ex.3-1 Compound (1-4) 1.5 mass % 93.4 135.6 Ex. 3-2 Compound (1-4) 2.5 mass% 99.7 155.6 Comp. — 100.0 100.0 Ex. 3-1

From Table 3, the use of the nonaqueous electrolytic solutions ofExamples 3-1 and 3-2 according to the invention resulted in lowerinitial capacity losses than when no esters of Formula (1) had beenadded (Comparative Example 3-1). Further, the batteries had a largeovercharge gas production as compared to Comparative Example 3-1,achieving higher safety. That is, the use of the inventive electrolyticsolutions makes it possible to obtain batteries having excellent initialbattery characteristics and excellent safety after overcharge durabilitytesting.

Examples 4-1 to 4-3 and Comparative Examples 4-1 to 4-3 Example 4-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (also written as “DEC”) (volume ratio 3:4:3)so that its concentration would be 1.2 mol/L. As additives, 2.0 mass %of monofluoroethylene carbonate (also written as “MP2”) and 2.0 mass %of vinylene carbonate (also written as “VC”) were dissolved therein,thus forming a basic electrolytic solution. Further, 0.5 mass % of thecompound (1-1) was added. In this manner, a nonaqueous electrolyticsolution of Example 4-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was allowed to stand at 45° C. for 72 hours.Thereafter, the battery was discharged at a constant current of 0.2 C to3.0 V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3.0 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff) and was dischargedat 0.2 C to 3.0 V, thereby determining the initial 0.2 C capacity. Here,1 C indicates a value of current at which a reference capacity of abattery is discharged in 1 hour. For example, 0.2 C indicates a currentthat is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was immersed in anethanol bath, and the volume of the battery before the testing ofhigh-temperature storage durability was measured based on the buoyancy(Archimedes' principle). Thereafter, the battery was stored at a hightemperature of 60° C. for 7 days. After being cooled sufficiently, thebattery was immersed in an ethanol bath, and its volume was measured.Based on the volume change before and after the high-temperature storagedurability test, the storage gas production was determined.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics and thehigh-temperature storage durability. The evaluation results are shown inTable 4 relative to the results of Comparative Example 4-1 taken as100.0%. The same applies hereinafter.

Example 4-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 4-1, except that 0.5 mass % of thecompound (1-1) used in the electrolytic solution of Example 4-1 wasreplaced by 1.0 mass % of the compound (1-1).

Example 4-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 4-1, except that 0.5 mass % of thecompound (1-1) used in the electrolytic solution of Example 4-1 wasreplaced by 1.0 mass % of the compound (1-1) and 3.0 mass % of1-phenyl-1,3,3-trimethylindane (also written as “MP12”).

Comparative Example 4-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 4-1, except that the electrolytic solutionof Example 4-1 did not contain the compound (1-1).

Comparative Example 4-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 4-3, except that the electrolytic solutionof Example 4-3 did not contain the compound (1-1).

Comparative Example 4-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 4-1, except that 0.5 mass % of thecompound (1-1) used in the electrolytic solution of Example 4-1 wasreplaced by 1.0 mass % of the compound (3-2).

TABLE 4 Initial 0.2 C Storage gas Additives capacity/% production/% Ex.4-1 Compound (1-1) 0.5 mass % 100.28 73.3 Ex. 4-2 Compound (1-1) 1.0mass % 100.42 81.9 Ex. 4-3 Compound (1-1) 1.0 mass % 100.37 99.0 MP123.0 mass % Comp. — 100.00 100.0 Ex. 4-1 Comp. MP12 3.0 mass % 100.32107.6 Ex. 4-2 Comp. Compound (3-2) 1.0 mass % 100.17 112.4 Ex. 4-3

From Table 4, the use of the nonaqueous electrolytic solutions ofExample 4-1, Example 4-2 and Example 4-3 according to the inventionresulted in higher initial 0.2 C capacities and smaller storage gasproductions during the high-temperature storage durability test thanwhen no esters of Formula (1) had been added (Comparative Example 4-1).That is, the use of the inventive electrolytic solutions makes itpossible to obtain batteries having excellent initial batterycharacteristics and excellent battery characteristics after the testingof high-temperature storage durability.

When the aromatic compound other than the compounds represented byFormula (1) or the aromatic compound outside the category of the estersrepresented by Formula (2) was used alone (Comparative Example 4-2 orComparative Example 4-3), the initial 0.2 C capacity was enhanced fromthe level in Comparative Example 4-1 but an increased amount of storagegas was generated as compared to Comparative Example 4-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

Examples 5-1 to 5-4 and Comparative Examples 5-1 to 5-3 Example 5-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 1.0 mass % of the compound (1-1) was added. In thismanner, a nonaqueous electrolytic solution of Example 5-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 21 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 12 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was allowed to stand at 45° C. for 72 hours.Thereafter, the battery was discharged at a constant current of 0.2 C to3 V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3.0 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff) and was dischargedat 0.2 C to 3.0 V, thereby determining the initial 0.2 C capacity.Further, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged at 0.5 C to 3.0 V, thereby determining the initial0.5 C capacity. The ratio of the initial 0.5 C capacity to the initial0.2 C capacity was obtained as the initial rate characteristic (%).

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability as anevaluation of battery characteristics, the nonaqueous electrolytesecondary battery was CC-CV charged at 25° C. and at a constant currentof 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, the battery wasovercharged at 45° C. and at a constant current of 0.2 C to 5.0 V. Afterthe battery had been cooled sufficiently, the open circuit voltage (OCV)was measured. The OCV after overcharging was thus obtained.

The OCV of a battery after overcharge testing mainly reflects thepotential of a positive electrode. Specifically, a lower OCV afterovercharging indicates a smaller charge depth in a positive electrode.Usually, the increase in the charge depth in a positive electroderesults in the dissolution of metal and the release of oxygen from thepositive electrode, thus initiating a thermal runaway of the battery.Thus, the safety of overcharged batteries may be ensured by thereduction of the OCV after overcharging.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the overcharge characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 5 relative to the results of ComparativeExample 5-1 taken as 100.0%. The OCV after overcharging is indicated asthe difference from the value in Comparative Example 5-1. The sameapplies hereinafter.

Example 5-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-1, except that the electrolytic solutionof Example 5-1 containing the compound (1-1) further contained 2.0 mass% of monofluoroethylene carbonate (hereinafter, also written as “MP2”).

Example 5-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-2, except that the electrolytic solutionof Example 5-2 containing the compound (1-1) and MP2 further contained2.0 mass % of vinylene carbonate (hereinafter, also written as “VC”).

Example 5-4

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-3, except that the compound (1-1) usedin the electrolytic solution of Example 5-3 was replaced by 1.0 mass %of the compound (1-4).

Comparative Example 5-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-1, except that the electrolytic solutionof Example 5-1 did not contain the compound (1-1).

Comparative Example 5-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-2, except that the electrolytic solutionof Example 5-2 did not contain the compound (1-1).

Comparative Example 5-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 5-4, except that the electrolytic solutionof Example 5-4 did not contain the compound (1-4).

TABLE 5 Initial 0.2 C Initial rate OCV after Additives capacity/%characteristic/% overcharging/mV Ex. 5-1 Compound (1-1) 1.0 mass %100.11 100.11 −153.8 Ex. 5-2 Compound (1-1) 1.0 mass % 98.67 101.29−164.0 MP2 2.0 mass % Ex. 5-3 Compound (1-1) 1.0 mass % 98.25 101.87−174.2 MP2 2.0 mass % VC 2.0 mass % Ex. 5-4 Compound (1-4) 1.0 mass %98.16 101.73 −159.8 MP2 2.0 mass % VC 2.0 mass % Comp. — 100.00 100.000.0 Ex. 5-1 Comp. MP2 2.0 mass % 98.45 101.25 −5.3 Ex. 5-2 Comp. MP2 2.0mass % 97.91 101.71 −23.6 Ex. 5-3 VC 2.0 mass %

From Table 5, the use of the nonaqueous electrolytic solution of Example5-1 according to the invention resulted in a higher initial 0.2 Ccapacity and a higher initial rate characteristic than when no esters ofFormula (1) had been added (Comparative Example 5-1). Further, ascompared to Comparative Example 5-1, the battery had a low OCV after thebattery had been overcharged after the testing of high-temperaturestorage durability, achieving higher safety. That is, the use of theinventive electrolytic solutions makes it possible to obtain batterieshaving excellent initial battery characteristics and excellent batterycharacteristics and safety after the testing of high-temperature storagedurability.

When the ester of Formula (1) was used together with thefluorine-containing cyclic carbonate or the cyclic carbonate having acarbon-carbon unsaturated bond (Examples 5-2 to 5-4), the initial 0.2 Ccapacity and the initial rate characteristic were enhanced as comparedto when no esters of Formula (1) had been added (Comparative Examples5-2 and 5-3). Further, as compared to Comparative Examples 5-2 and 5-3,the batteries had a low OCV after the batteries had been overchargedafter the testing of high-temperature storage durability, achievinghigher safety. That is, the use of the inventive electrolytic solutionsmakes it possible to obtain batteries having excellent initial batterycharacteristics and excellent battery characteristics and safety afterthe testing of high-temperature storage durability.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (1) and fluorine-containing cyclic carbonates orcyclic carbonates having a carbon-carbon unsaturated bond.

Example 6-1 and Comparative Examples 6-1 to 6-3 Example 6 [Preparationof Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % oftriethyl phosphonoacetate (also written as “MP1”) as additives wereadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 6-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3V. Next, the battery was CC-CV charged at 0.2 C to 4.40 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas immersed in an ethanol bath and the volume was measured. The initialgas production was obtained by determining the change from the initialvolume of the battery. Thereafter, the battery was CC-CV charged at 0.2C to 4.40 V (0.05 C cutoff) and was discharged at 0.2 C to 3 V, therebydetermining the initial 0.2 C capacity. Further, the battery was CC-CVcharged at 0.2 C to 4.40 V (0.05 C cutoff) and was discharged at 0.5 Cto 3 V, thereby determining the initial 0.5 C capacity. The ratio of theinitial 0.5 C capacity to the initial 0.2 C capacity was obtained as theinitial rate characteristic (%).

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.40 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.2 C to 3 V. The capacity was obtainedas the recovered 0.2 C capacity.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 6 relative to the results of ComparativeExample 6-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 6-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 6-1, except that the electrolytic solutionof Example 6-1 did not contain the compound (2-1) and MP1.

Comparative Example 6-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 6-1, except that the electrolytic solutionof Example 6-1 did not contain MP1.

Comparative Example 6-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 6-1, except that the electrolytic solutionof Example 6-1 did not contain the compound (2-1).

TABLE 6 Initial gas Initial rate Recovered 0.2 C Additives production/%characteristic/% capacity/% Ex. 6-1 Compound (2-1) 0.5 mass % 136.0100.38 102.14 MP1 0.5 mass % Comp. — 100.0 100.00 100.00 Ex. 6-1 Comp.Compound (2-1) 0.5 mass % 140.0 100.25 100.74 Ex. 6-2 Comp. MP1 0.5 mass% 158.7 100.34 101.22 Ex. 6-3

From Table 6, the use of the nonaqueous electrolytic solution of Example6-1 according to the invention resulted in a high initial ratecharacteristic and a high recovered 0.2 C capacity after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no phosphonate esters had been added at the same time(Comparative Example 6-1). That is, the use of the inventiveelectrolytic solutions makes it possible to obtain batteries havingexcellent initial battery characteristics and excellent batterycharacteristics after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 6-2),the battery exhibited an enhanced initial rate characteristic ascompared to Comparative Example 6-1 but the initial gas production wasincreased from the level in Example 6-1. While the recovered 0.2 Ccapacity was enhanced as compared to Comparative Example 6-1, theimprovement was smaller than that obtained in Example 6-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

When the phosphonate ester was used alone (Comparative Example 6-3), thebattery exhibited an enhanced initial rate characteristic as compared toComparative Example 6-1 but the initial gas production was increasedfrom the level in Example 6-1. While the recovered 0.2 C capacity wasenhanced as compared to Comparative Example 6-1, the improvement wassmaller than that obtained in Example 6-1. It is thus clear that the useof the inventive electrolytic solutions provides excellent batterycharacteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and phosphonate esters.

Example 7-1 and Comparative Examples 7-1 to 7-3 Example 7-1 [Preparationof Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % ofmonofluoroethylene carbonate (also written as “MP2”) as additives wereadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 7-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff). The nonaqueouselectrolyte secondary battery was then immersed in an ethanol bath andthe volume was measured. The initial gas production was obtained bydetermining the change from the initial volume of the battery.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath, and the volume of the battery beforeovercharging was measured based on the buoyancy. Thereafter, the batterywas overcharged at 45° C. and at a constant current of 0.2 C to 5.0 V.After being cooled sufficiently, the battery was immersed in an ethanolbath and its volume was measured. The change in battery volume frombefore the overcharging was obtained as the overcharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the overcharge characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 7 relative to the results of ComparativeExample 7-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 7-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 7-1, except that the electrolytic solutionof Example 7-1 did not contain the compound (2-1) and MP2.

Comparative Example 7-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 7-1, except that the electrolytic solutionof Example 7-1 did not contain MP2.

Comparative Example 7-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 7-1, except that the electrolytic solutionof Example 7-1 did not contain the compound (2-1).

TABLE 7 Overcharge Initial gas gas production/ production/ Additives % %Ex. 7-1 Compound (2-1) 0.5 mass % 74.7 333.3 MP2 0.5 mass % Comp. Ex.7-1 — 100.0 100.0 Comp. Ex. 7-2 Compound (2-1) 0.5 mass % 140.0 311.1Comp. Ex. 7-3 MP2 0.5 mass % 84.0 66.7

From Table 7, the use of the nonaqueous electrolytic solution of Example7-1 according to the invention resulted in a small initial gasproduction and a large overcharge gas production after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no fluorine-containing cyclic carbonates had been addedat the same time (Comparative Example 7-1). That is, the use of theinventive electrolytic solutions makes it possible to obtain batterieshaving excellent initial battery characteristics and excellentovercharge characteristics evaluated after the testing ofhigh-temperature storage durability.

The use of the ester of Formula (2) alone (Comparative Example 7-2)resulted in an increase in initial gas production from the level inComparative Example 7-1. While the overcharge gas production wasincreased as compared to Comparative Example 7-1, the improvement wassmaller than that obtained in Example 7-1. It is thus clear that the useof the inventive electrolytic solutions provides excellent batterycharacteristics.

While the use of the fluorine-containing cyclic carbonate alone(Comparative Example 7-3) reduced the initial gas production as comparedto the level in Comparative Example 7-1, the improvement was smallerthan that obtained in Example 7-1. Further, the overcharge gasproduction was smaller than Comparative Example 7-1. It is thus clearthat the use of the inventive electrolytic solutions provides excellentbattery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and fluorine-containing cyclic carbonates.

Example 8-1 and Comparative Examples 8-1 to 8-3 Example 8-1 [Preparationof Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % oflithium fluorosulfonate (also written as “MP3”) as additives were addedto the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 8-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff) and was discharged at 1C to 3.0 V, thereby determining the initial 1 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.2 C to 3.0 V. The capacity wasobtained as the recovered 0.2 C capacity.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 8 relative to the results of ComparativeExample 8-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 8-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 8-1, except that the electrolytic solutionof Example 8-1 did not contain the compound (2-1) and MP3.

Comparative Example 8-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 8-1, except that the electrolytic solutionof Example 8-1 did not contain MP3.

Comparative Example 8-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 8-1, except that the electrolytic solutionof Example 8-1 did not contain the compound (2-1).

TABLE 8 Recovered Initial 1 C 0.2 C Additives capacity/% capacity/% Ex.8-1 Compound (2-1) 0.5 mass % 100.69 100.79 MP3 0.5 mass % Comp. Ex. 8-1— 100.00 100.00 Comp. Ex. 8-2 Compound (2-1) 0.5 mass % 100.17 100.74Comp. Ex. 8-3 MP3 0.5 mass % 100.59 100.52

From Table 8, the use of the nonaqueous electrolytic solution of Example8-1 according to the invention resulted in a high initial 1 C capacityand a high recovered 0.2 C capacity after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no fluorosulfonate salts had been added at the same time(Comparative Example 8-1). That is, the use of the inventiveelectrolytic solutions makes it possible to obtain batteries havingexcellent initial battery characteristics and excellent batterycharacteristics after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 8-2),the battery exhibited an enhanced initial 1 C capacity as compared toComparative Example 8-1 but the improvement was smaller than thatobtained in Example 8-1. While the recovered 0.2 C capacity was enhancedas compared to Comparative Example 8-1, the improvement was smaller thanthat obtained in Example 8-1. It is thus clear that the use of theinventive electrolytic solutions provides excellent batterycharacteristics.

When the fluorosulfonate salt was used alone (Comparative Example 8-3),the battery exhibited an enhanced initial 1 C capacity as compared toComparative Example 8-1 but the improvement was smaller than thatobtained in Example 8-1. While the recovered 0.2 C capacity was enhancedas compared to Comparative Example 8-1, the improvement was smaller thanthat obtained in Example 8-1. It is thus clear that the use of theinventive electrolytic solutions provides excellent batterycharacteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and fluorosulfonate salts.

Example 9-1 and Comparative Examples 9-1 to 9-3 Example 9-1 [Preparationof Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % of1,3-bis(isocyanatomethyl)cyclohexane (also written as “MP4”) asadditives were added to the basic electrolytic solution. In this manner,a nonaqueous electrolytic solution of Example 9-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath and the volume was measured. The initial gasproduction was obtained by determining the change from the initialvolume of the battery.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.2 C to 3.0 V. The capacity wasobtained as the recovered 0.2 C capacity.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 9 relative to the results of ComparativeExample 9-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 9-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 9-1, except that the electrolytic solutionof Example 9-1 did not contain the compound (2-1) and MP4.

Comparative Example 9-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 9-1, except that the electrolytic solutionof Example 9-1 did not contain MP4.

Comparative Example 9-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 9-1, except that the electrolytic solutionof Example 9-1 did not contain the compound (2-1).

TABLE 9 Initial gas Recovered production/ 0.2 C Additives % capacity/%Ex. 9-1 Compound (2-1) 0.5 mass % 66.7 100.80 MP4 0.5 mass % Comp. Ex.9-1 — 100.0 100.00 Comp. Ex. 9-2 Compound (2-1) 0.5 mass % 140.0 100.74Comp. Ex. 9-3 MP4 0.5 mass % 81.3 100.12

From Table 9, the use of the nonaqueous electrolytic solution of Example9-1 according to the invention resulted in a small initial gasproduction and a high recovered 0.2 C capacity after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no isocyanate group-containing organic compounds hadbeen added at the same time (Comparative Example 9-1). That is, the useof the inventive electrolytic solutions makes it possible to obtainbatteries having excellent initial battery characteristics and excellentbattery characteristics after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 9-2),the initial gas production was increased from the level in ComparativeExample 9-1. While the recovered 0.2 C capacity was enhanced as comparedto Comparative Example 9-1, the improvement was smaller than thatobtained in Example 9-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

When the isocyanate group-containing organic compound was used alone(Comparative Example 9-3), the initial gas production was decreased fromthe level in Comparative Example 9-1 but was larger than that in Example9-1. While the recovered 0.2 C capacity was enhanced as compared toComparative Example 9-1, the improvement was smaller than that obtainedin Example 9-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and isocyanate group-containing organiccompounds.

Example 10-1 and Comparative Examples 10-1 to 10-3 Example 10-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % ofadiponitrile (also written as “MP5”) as additives were added to thebasic electrolytic solution. In this manner, a nonaqueous electrolyticsolution of Example 10-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethyicellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff) and was discharged at0.5 C to 3.0 V, and the ratio of the discharge capacity to the chargecapacity obtained during this process was determined as the initial 0.5C efficiency (%).

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.05 C to 3.0 V. The ratio of thedischarge capacity to the charge capacity obtained during this processwas determined as the recovered 0.05 C efficiency (%).

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 10 relative to the results of ComparativeExample 10-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 10-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 10-1, except that the electrolyticsolution of Example 10-1 did not contain the compound (2-1) and MP5.

Comparative Example 10-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 10-1, except that the electrolyticsolution of Example 10-1 did not contain MP5.

Comparative Example 10-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 10-1, except that the electrolyticsolution of Example 10-1 did not contain the compound (2-1).

TABLE 10 Initial Recovered 0.5 C 0.05 C efficiency/ efficiency/Additives % % Ex. 10-1 Compound (2-1) 0.5 mass % 100.0 100.8 MP5 0.5mass % Comp. Ex. — 100.0 100.0 10-1 Comp. Ex. Compound (2-1) 0.5 mass %99.9 99.9 10-2 Comp. Ex. MP5 0.5 mass % 99.9 100.5 10-3

From Table 10, the use of the nonaqueous electrolytic solution ofExample 10-1 according to the invention prevented a decrease in initial0.5 C efficiency and provided an excellent recovered 0.05 C efficiencyafter the testing of high-temperature storage durability as compared towhen no esters of Formula (2) and no cyano group-containing organiccompounds had been added at the same time (Comparative Example 10-1).That is, the use of the inventive electrolytic solutions makes itpossible to obtain batteries having excellent initial batterycharacteristics and excellent battery characteristics after the testingof high-temperature storage durability.

When the ester of Formula (2) was used alone (Comparative Example 10-2),the initial 0.5 C efficiency was decreased from the level in ComparativeExample 10-1. Further, the recovered 0.05 C efficiency was lower thanthat in Comparative Example 10-1. It is thus clear that the use of theinventive electrolytic solutions provides excellent batterycharacteristics.

When the cyano group-containing organic compound was used alone(Comparative Example 10-3), the initial 0.5 C efficiency was decreasedfrom the level in Comparative Example 10-1. While the recovered 0.05 Cefficiency was enhanced as compared to Comparative Example 10-1, theimprovement was smaller than that obtained in Example 10-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and cyano group-containing organic compounds.

Example 11-1 and Comparative Examples 11-1 to 11-3 Example 11-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % ofhexamethyldisilane (also written as “MP6”) as additives were added tothe basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 11-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff) and was discharged at 1C to 3.0 V, thereby determining the initial 1 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.05 C to 3.0 V, and the capacity wasobtained as the recovered 0.05 C capacity. Further, the ratio of thedischarge capacity to the charge capacity obtained during this processwas determined as the recovered 0.05 C efficiency (%).

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 11 relative to the results of ComparativeExample 11-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 11-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 11-1, except that the electrolyticsolution of Example 11-1 did not contain the compound (2-1) and MP6.

Comparative Example 11-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 11-1, except that the electrolyticsolution of Example 11-1 did not contain MP6.

Comparative Example 11-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 11-1, except that the electrolyticsolution of Example 11-1 did not contain the compound (2-1).

TABLE 11 Initial Recovered Recovered 1 C 0.05 C 0.05 C capacity/capacity/ efficiency/ Additives % % % Ex. 11-1 Compound 0.5 100.50100.65 100.5 (2-1) mass % MP6 0.5 mass % Comp. — 100.00 100.00 100.0 Ex.11-1 Comp. Compound 0.5 100.17 100.63 99.9 Ex. 11-2 (2-1) mass % Comp.MP6 0.5 100.35 99.96 100.4 Ex. 11-3 mass %

From Table 11, the use of the nonaqueous electrolytic solution ofExample 11-1 according to the invention resulted in a high initial 1 Ccapacity and also resulted in an excellent recovered 0.05 capacity andan excellent recovered 0.05 C efficiency after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no silicon-containing compounds had been added at thesame time (Comparative Example 11-1). That is, the use of the inventiveelectrolytic solutions makes it possible to obtain batteries havingexcellent initial battery characteristics and excellent batterycharacteristics after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 11-2),the initial 1 C capacity was enhanced as compared to Comparative Example11-1 but the improvement was smaller than that obtained in Example 11-1.Further, the recovered 0.05 C capacity was enhanced as compared toComparative Example 11-1 but was lower than that in Example 11-1.Furthermore, the recovered 0.05 C efficiency was lower than that inComparative Example 11-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

When the silicon-containing compound was used alone (Comparative Example11-3), the initial 1 C capacity was enhanced as compared to ComparativeExample 11-1 but the improvement was smaller than that obtained inExample 11-1. Further, the recovered 0.05 C capacity was decreased fromthe level in Comparative Example 11-1. While the recovered 0.05 Cefficiency was enhanced as compared to Comparative Example 11-1, theimprovement was smaller than that obtained in Example 11-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and silicon-containing compounds.

Example 12-1 and Comparative Examples 12-1 to 12-3 Example 12-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % oflithium tetrafluoroborate (also written as “MP7”) as additives wereadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 12-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.40 V (0.05 C cutoff) and was discharged at 1C to 3.0 V, thereby determining the initial 1 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.4 V (0.05 C cutoff) and was dischargedagain at 0.2 C to 3.0 V. The ratio of the discharge capacity to thecharge capacity obtained during this process was determined as therecovered 0.2 C efficiency (%).

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability and theevaluation of battery characteristics, the nonaqueous electrolytesecondary battery was CC-CV charged at 25° C. and at a constant currentof 0.2 C to 4.4 V (0.05 C cutoff). Thereafter, the battery wasovercharged at 45° C. and at a constant current of 0.2 C to 5.0 V. Afterthe battery had been cooled sufficiently, the open circuit voltage (OCV)was measured. The OCV after overcharging was thus obtained.

The OCV of a battery after overcharge testing mainly reflects thepotential of a positive electrode. Specifically, a lower OCV afterovercharging indicates a smaller charge depth in a positive electrode.Usually, the increase in the charge depth in a positive electroderesults in the dissolution of metal and the release of oxygen from thepositive electrode, thus initiating a thermal runaway of the battery.Thus, the safety of overcharged batteries may be ensured by thereduction of the OCV after overcharging.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability, the battery characteristics afterthe testing of high-temperature storage durability, and the overchargecharacteristics after the testing of high-temperature storagedurability. The evaluation results are shown in Table 12 relative to theresults of Comparative Example 12-1 taken as 100.0%. The OCV afterovercharging is indicated as the difference from the value inComparative Example 12-1. The same applies hereinafter.

Comparative Example 12-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 12-1, except that the electrolyticsolution of Example 12-1 did not contain the compound (2-1) and MP7.

Comparative Example 12-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 12-1, except that the electrolyticsolution of Example 12-1 did not contain MP7.

Comparative Example 12-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 12-1, except that the electrolyticsolution of Example 12-1 did not contain the compound (2-1).

TABLE 12 Initial 1 C Recovered OCV after capacity/ 0.2 C overcharging/Additives % efficiency/% mV Ex. 12-1 Compound 0.5 100.36 100.1 −27.5(2-1) mass % MP7 0.5 mass % Comp. — 100.00 100.0 0.0 Ex. 12-1 Comp.Compound 0.5 100.17 100.1 −25.4 Ex. 12-2 (2-1) mass % Comp. MP7 0.5100.24 99.7 −8.8 Ex. 12-3 mass %

From Table 12, the use of the nonaqueous electrolytic solution ofExample 12-1 according to the invention resulted in a high initial 1 Ccapacity and a high recovered 0.2 C efficiency after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no borate salts had been added at the same time(Comparative Example 12-1). Further, as compared to Comparative Example12-1, the battery had a low OCV after the battery had been overchargedafter the testing of high-temperature storage durability, achievinghigher safety. That is, the use of the inventive electrolytic solutionsmakes it possible to obtain batteries having excellent initial batterycharacteristics and excellent battery characteristics and safety afterthe testing of high-temperature storage durability.

When the ester of Formula (2) was used alone (Comparative Example 12-2),the initial 1 C capacity was enhanced as compared to Comparative Example12-1 but the improvement was smaller than that obtained in Example 12-1.The OCV after overcharging was lower than that in Comparative Example12-1 but compared unfavorably to that in Example 12-1. It is thus clearthat the use of the inventive electrolytic solutions provides excellentbattery characteristics.

When the borate salt was used alone (Comparative Example 12-3), theinitial 1 C capacity was enhanced as compared to Comparative Example12-1 but the improvement was smaller than that obtained in Example 12-1.Further, the recovered 0.2 C efficiency was lower than that inComparative Example 12-1. The OCV after overcharging was lower than thatin Comparative Example 12-1 but compared unfavorably to that in Example12-1. It is thus clear that the use of the inventive electrolyticsolutions provides excellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and borate salts.

Example 13-1 and Comparative Examples 13-1 to 13-3 Example 13-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 3:4:3) so that itsconcentration would be 1.0 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % of1,3-propanesultone (also written as “MP8”) as additives were added tothe basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 13-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff) and was discharged at0.2 C to 3.0 V, and the ratio of the discharge capacity to the chargecapacity obtained during this process was determined as the initial 0.2C efficiency (%).

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.2 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.2 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath, and the volume of the battery beforeovercharging was measured based on the buoyancy (Archimedes' principle).Thereafter, the battery was overcharged at 45° C. and at a constantcurrent of 0.5 C to 5.0 V. After being cooled sufficiently, the batterywas immersed in an ethanol bath and its volume was measured. The changein battery volume from before the overcharging was obtained as theovercharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging, the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the overcharge characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 13 relative to the results of ComparativeExample 13-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 13-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 13-1, except that the electrolyticsolution of Example 13-1 did not contain the compound (2-1) and MP8.

Comparative Example 13-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 13-1, except that the electrolyticsolution of Example 13-1 did not contain MP8.

Comparative Example 13-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 13-1, except that the electrolyticsolution of Example 13-1 did not contain the compound (2-1).

TABLE 13 Initial Overcharge 0.2 C gas efficiency/ production/ Additives% % Ex. 13-1 Compound 0.5 mass % 100.2 466.7 (2-1) MP8 0.5 mass % Comp.Ex. 13-1 — 100.0 100.0 Comp. Ex. 13-2 Compound 0.5 mass % 100.0 366.7(2-1) Comp. Ex. 13-3 MP8 0.5 mass % 100.1 133.3

From Table 13, the use of the nonaqueous electrolytic solution ofExample 13-1 according to the invention resulted in an excellent initial0.2 C efficiency and a large overcharge gas production after the testingof high-temperature storage durability as compared to when no esters ofFormula (2) and no sulfur-containing organic compounds had been added atthe same time (Comparative Example 13-1). That is, the use of theinventive electrolytic solutions makes it possible to obtain batterieshaving excellent initial battery characteristics and excellentovercharge characteristics evaluated after the testing ofhigh-temperature storage durability.

When the ester of Formula (2) was used alone (Comparative Example 13-2),the initial 0.2 C efficiency was unchanged from the result obtained inComparative Example 13-1. While the overcharge gas production wasincreased as compared to Comparative Example 13-1, the improvement wassmaller than that obtained in Example 13-1. It is thus clear that theuse of the inventive electrolytic solutions provides excellent batterycharacteristics.

When the sulfur-containing organic compound was used alone (ComparativeExample 13-3), the initial 0.2 C efficiency was enhanced as compared toComparative Example 13-1 but the improvement was smaller than thatobtained in Example 13-1. While the overcharge gas production wasincreased as compared to Comparative Example 13-1, the improvement wassmaller than that obtained in Example 13-1. It is thus clear that theuse of the inventive electrolytic solutions provides excellent batterycharacteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and sulfur-containing organic compounds.

Example 14-1 to 14-2 and Comparative Examples 14-1 to 14-6 Example 14-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 3:4:3) so that itsconcentration would be 1.0 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % ofethyl propionate (also written as “MP9”) as additives were added to thebasic electrolytic solution. In this manner, a nonaqueous electrolyticsolution of Example 14-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. In this manner, initial battery characteristics were stabilized.Thereafter, the battery was CC-CV charged at 0.2 C to 4.4 V (0.05 Ccutoff) and was discharged at 0.2 C to 3.0 V, thereby determining theinitial 0.2 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.4 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.2 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath, and the volume of the battery beforeovercharging was measured based on the buoyancy (Archimedes' principle).Thereafter, the battery was overcharged at 45° C. and at a constantcurrent of 0.5 C to 5.0 V. After being cooled sufficiently, the batterywas immersed in an ethanol bath and its volume was measured. The changein battery volume from before the overcharging was obtained as theovercharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging, the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the overcharge characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 14 relative to the results of ComparativeExample 14-1 taken as 100.0%. The same applies hereinafter.

Example 14-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-1, except that MP9 used in theelectrolytic solution of Example 14-1 was replaced by 0.5 mass % ofn-propyl propionate (also written as “MP9′”).

Comparative Example 14-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-1, except that the electrolyticsolution of Example 14-1 did not contain the compound (2-1) and MP9.

Comparative Example 14-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-1, except that the electrolyticsolution of Example 14-1 did not contain MP9.

Comparative Example 14-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-1, except that the electrolyticsolution of Example 14-1 did not contain the compound (2-1).

Comparative Example 14-4

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-2, except that the electrolyticsolution of Example 14-2 did not contain the compound (2-1).

Comparative Example 14-5

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 14-1, except that MP9 used in theelectrolytic solution of Example 14-1 was replaced by 0.5 mass % ofmethyl propionate (also written as “MP”).

Comparative Example 14-6

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Comparative Example 14-5, except that theelectrolytic solution of Comparative Example 14-5 did not contain thecompound (2-1).

TABLE 14 Initial Overcharge 0.2 C gas Additives capacity/% production/%Ex. 14-1 Compound (2-1) 0.5 mass % 100.46 780.0 MP9 0.5 mass % Ex. 14-2Compound (2-1) 0.5 mass % 100.30 800.0 MP9′ 0.5 mass % Comp. Ex. 14-1 —100.00 100.0 Comp. Ex. 14-2 Compound (2-1) 0.5 mass % 99.66 580.0 Comp.Ex. 14-3 MP9 0.5 mass % 99.83 20.0 Comp. Ex. 14-4 MP9′ 0.5 mass % 100.2260.0 Comp. Ex. 14-5 Compound (2-1) 0.5 mass % 99.58 680.0 MP 0.5 mass %Comp. Ex. 14-6 MP 0.5 mass % 100.27 60.0

From Table 14, the use of the nonaqueous electrolytic solutions ofExamples 14-1 and 14-2 according to the invention resulted in excellentinitial 0.2 C capacities and large overcharge gas productions after thetesting of high-temperature storage durability as compared to when noaromatic carboxylate esters of Formula (2) and no carboxylate esters ofFormula (3) had been added at the same time (Comparative Example 14-1).That is, the use of the inventive electrolytic solutions makes itpossible to obtain batteries having excellent initial batterycharacteristics and excellent overcharge characteristics evaluated afterthe testing of high-temperature storage durability.

When the aromatic carboxylate ester of Formula (2) was used alone(Comparative Example 14-2), the initial 0.2 C capacity was decreasedfrom the level in Comparative Example 14-1. While the overcharge gasproduction was increased as compared to Comparative Example 14-1, theimprovement was smaller than that obtained in Examples 14-1 and 14-2. Itis thus clear that the use of the inventive electrolytic solutionsprovides excellent battery characteristics.

When the carboxylate ester of Formula (3) was used alone (ComparativeExamples 14-3 and 14-4), the overcharge gas productions were decreasedfrom the level in Comparative Example 14-1 and were far below the amountobtained in Example 14-1. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

When the aromatic compound outside the category of the carboxylateesters represented by Formula (3), and the aromatic carboxylate esterrepresented by Formula (2) were added at the same time (ComparativeExample 14-5), the initial 0.2 C capacity was decreased from the levelin Comparative Example 14-1 and was far below the capacities obtained inExamples 14-1 and 14-2. It is thus clear that the use of the inventiveelectrolytic solutions provides excellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe aromatic carboxylate esters of Formula (2) and the carboxylateesters of Formula (3).

Example 15-1 and Comparative Examples 15-1 to 15-3 Example 15-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 3:4:3) so that itsconcentration would be 1.0 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % oflithium difluorophosphate (also written as “MP10”) as additives wereadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 15-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff). The nonaqueouselectrolyte secondary battery was then immersed in an ethanol bath, andthe volume was measured. The initial gas production was obtained bydetermining the change in volume from the initial volume of the battery.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.2 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Overcharge Characteristics after Testing ofHigh-Temperature Storage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.2 V (0.05 C cutoff). The battery was thenimmersed in an ethanol bath, and the volume of the battery beforeovercharging was measured based on the buoyancy (Archimedes' principle).Thereafter, the battery was overcharged at 45° C. and at a constantcurrent of 0.5 C to 5.0 V. After being cooled sufficiently, the batterywas immersed in an ethanol bath and its volume was measured. The changein battery volume from before the overcharging was obtained as theovercharge gas production.

In such types of batteries in which a safety valve is operated upon adetection of an unusual increase in internal pressure due toabnormalities such as overcharging, the generation of a larger amount ofovercharge gas is more advantageous because the safety valve can beoperated early and consequently the battery safety in the event ofovercharging can be ensured.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the overcharge characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 15 relative to the results of ComparativeExample 15-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 15-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 15-1, except that the electrolyticsolution of Example 15-1 did not contain the compound (2-1) and MP10.

Comparative Example 15-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 15-1, except that the electrolyticsolution of Example 15-1 did not contain MP10.

Comparative Example 15-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 15-1, except that the electrolyticsolution of Example 15-1 did not contain the compound (2-1).

TABLE 15 Initial gas Overcharge production/ gas Additives % production/%Ex. 15-1 Compound (2-1) 0.5 mass % 73.5 400.0 MP10 0.5 mass % Comp. Ex.15-1 — 100.0 100.0 Comp. Ex. 15-2 Compound (2-1) 0.5 mass % 129.9 366.7Comp. Ex. 15-3 MP10 0.5 mass % 79.5 33.3

From Table 15, the use of the nonaqueous electrolytic solution ofExample 15-1 according to the invention resulted in a small initial gasproduction and a large overcharge gas production after the testing ofhigh-temperature storage durability as compared to when no esters ofFormula (2) and no lithium difluorophosphate had been added at the sametime (Comparative Example 15-1). That is, the use of the inventiveelectrolytic solutions makes it possible to obtain batteries havingexcellent initial battery characteristics and excellent overchargecharacteristics evaluated after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 15-2),the initial gas production was increased from the level in ComparativeExample 15-1. While the overcharge gas production was increased ascompared to Comparative Example 15-1, the improvement was smaller thanthat obtained in Example 15-1. It is thus clear that the use of theinventive electrolytic solutions provides excellent batterycharacteristics.

When the lithium difluorophosphate was used alone (Comparative Example15-3), the initial gas production was small as compared to ComparativeExample 15-1 but the improvement was smaller than that obtained inExample 15-1. Further, the overcharge gas production was decreased fromthe level in Comparative Example 15-1. It is thus clear that the use ofthe inventive electrolytic solutions provides excellent batterycharacteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and lithium difluorophosphate.

Example 16-1 and Comparative Examples 16-1 to 16-3 Example 16-1[Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC) (volume ratio 3:4:3) so that itsconcentration would be 1.0 mol/L, thus forming a basic electrolyticsolution. Further, 0.5 mass % of the compound (2-1) and 0.5 mass % oflithium bis(oxalato)borate (also written as “MP11”) as additives wereadded to the basic electrolytic solution. In this manner, a nonaqueouselectrolytic solution of Example 16-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

The nonaqueous electrolyte secondary battery was immersed in an ethanolbath, and the initial volume of the battery was measured based on thebuoyancy (Archimedes' principle). While being pressed between glassplates, the battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was discharged at a constant current of 0.2 C to 3.0V. Next, the battery was CC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff)and was discharged again at 0.2 C to 3.0 V. In this manner, initialbattery characteristics were stabilized. Thereafter, the battery wasCC-CV charged at 0.2 C to 4.2 V (0.05 C cutoff) and was discharged at0.2 C to 3.0 V, and the ratio of the discharge capacity to the chargecapacity obtained during this process was determined as the initial 0.2C efficiency (%). Subsequently, the battery was CC-CV charged at 0.2 Cto 4.4 V (0.05 C cutoff). The nonaqueous electrolyte secondary batterywas then immersed in an ethanol bath, and the volume was measured. Theinitial gas production was obtained by determining the change in volumefrom the initial volume of the battery.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.20 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 85° C. for 1 day. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.2 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.2 C to 3.0 V. The ratio of thedischarge capacity to the charge capacity obtained during this processwas determined as the recovered 0.2 C efficiency (%).

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 16 relative to the results of ComparativeExample 16-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 16-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 16-1, except that the electrolyticsolution of Example 16-1 did not contain the compound (2-1) and MP11.

Comparative Example 16-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 16-1, except that the electrolyticsolution of Example 16-1 did not contain MP11.

Comparative Example 16-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 16-1, except that the electrolyticsolution of Example 16-1 did not contain the compound (2-1).

TABLE 16 Initial Recovered 0.2 C 0.2 C efficiency/ efficiency/ Additives% % Ex. 16-1 Compound (2-1) 0.5 mass % 100.2 100.2 MP11 0.5 mass % Comp.Ex. 16-1 — 100.0 100.0 Comp. Ex. 16-2 Compound (2-1) 0.5 mass % 100.0100.0 Comp. Ex. 16-3 MP11 0.5 mass % 100.0 100.0

From Table 16, the use of the nonaqueous electrolytic solution ofExample 16-1 according to the invention resulted in an excellent initial0.2 C efficiency and an excellent recovered 0.2 C efficiency after thetesting of high-temperature storage durability as compared to when noesters of Formula (2) and no oxalate salts had been added at the sametime (Comparative Example 16-1). That is, the use of the inventiveelectrolytic solutions makes it possible to obtain batteries havingexcellent initial battery characteristics and excellent batterycharacteristics evaluated after the testing of high-temperature storagedurability.

When the ester of Formula (2) was used alone (Comparative Example 16-2),the initial 0.2 C efficiency was unchanged from the result obtained inComparative Example 16-1. Further, the recovered 0.2 C efficiency wasthe same as the result obtained in Comparative Example 16-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

When the oxalate salt was used alone (Comparative Example 16-3), theinitial 0.2 C efficiency was unchanged from the result obtained inComparative Example 16-1. Further, the recovered 0.2 C efficiency wasthe same as the result obtained in Comparative Example 16-1. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and oxalate salts.

Examples 17-1 and 17-2, and Comparative Examples 17-1 to 17-4 Example17-1 [Preparation of Nonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L. As an additive, 5.0 mass % ofmonofluoroethylene carbonate (MP2) was dissolved, thus forming a basicelectrolytic solution. Further, 1.0 mass % of the compound (2-1) wasadded. In this manner, a nonaqueous electrolytic solution of Example17-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 15 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 10 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolypropylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was allowed to stand at 45° C. for 72 hours.Thereafter, the battery was discharged at a constant current of 0.2 C to3.0 V. Next, the battery was CC-CV charged at 0.2 C to 4.35 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3.0 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas CC-CV charged at 0.2 C to 4.35 V (0.05 C cutoff) and was dischargedat 0.2 C to 3.0 V, thereby determining the initial 0.2 C capacity.Further, the battery was CC-CV charged at 0.2 C to 4.35 V (0.05 Ccutoff) and was discharged at 0.5 C to 3.0 V, thereby determining theinitial 0.5 C capacity. The ratio of the initial 0.5 C capacity to theinitial 0.2 C capacity was determined as the initial rate characteristic(%).

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.35 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 60° C. for 7 days. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V.

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.35 V (0.05 C cutoff). Thereafter, thebattery was discharged again at 0.2 C to 3.0 V, thereby determining therecovered 0.2 C capacity. Further, the battery was CC-CV charged at 0.2C to 4.35 V (0.05 C cutoff) and was discharged at 0.5 C to 3.0 V,thereby determining the recovered 0.5 C capacity. The ratio of therecovered 0.5 C capacity to the recovered 0.2 C capacity was determinedas the rate characteristic (%) after storage.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 17 relative to the results of ComparativeExample 17-1 taken as 100.0%. The same applies hereinafter.

Example 17-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 17-1, except that 3.0 mass % ofl-phenyl-1,3,3-trimethylindane (also written as “MP12”) was furtheradded to the electrolytic solution of Example 17-1.

Comparative Example 17-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 17-1, except that the electrolyticsolution of Example 17-1 did not contain the compound (2-1).

Comparative Example 17-2

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 17-2, except that the electrolyticsolution of Example 17-2 did not contain the compound (2-1).

Comparative Example 17-3

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 17-1, except that the compound (2-1) usedin the electrolytic solution of Example 17-1 was replaced by 1.0 mass %of the compound (3-1).

Comparative Example 17-4

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 17-2, except that the compound (2-1) usedin the electrolytic solution of Example 17-2 was replaced by 1.0 mass %of the compound (3-1).

TABLE 17 Rate Initial rate characteristic characteristic/ afterAdditives % storage/% Ex. 17-1 Compound 1.0 mass % 100.03 100.02 (2-1)Ex. 17-2 Compound 1.0 mass % 100.09 100.81 (2-1) MP12 3.0 mass % Comp.Ex. 17-1 — 100.00 100.00 Comp. Ex. 17-2 MP12 3.0 mass % 99.98 100.69Comp. Ex. 17-3 Compound 1.0 mass % 100.03 99.76 (3-1) Comp. Ex. 17-4Compound 1.0 mass % 100.03 94.86 (3-1) MP12 3.0 mass %

From Table 17, the use of the nonaqueous electrolytic solution ofExample 17-1 according to the invention resulted in an excellent initialrate characteristic and an excellent rate characteristic evaluated afterthe battery had been stored during the testing of high-temperaturestorage durability, as compared to when the fluorine-containing cycliccarbonate had been used alone (Comparative Example 17-1). That is, theuse of the inventive electrolytic solutions makes it possible to obtainbatteries having excellent initial battery characteristics and excellentbattery characteristics evaluated after the testing of high-temperaturestorage durability.

Further, the use of the nonaqueous electrolytic solution of Example 17-2according to the invention resulted in an excellent initial ratecharacteristic and an excellent rate characteristic evaluated after thebattery had been stored during the testing of high-temperature storagedurability, as compared to when no esters of Formula (2) and no aromaticcompounds other than those of Formula (2) had been added at the sametime (Comparative Example 17-1). It should be noted that the improvementobtained in this case was higher than that obtained in Example 17-1. Asdemonstrated here, the use of the inventive electrolytic solutions makesit possible to obtain batteries having highly excellent initial batterycharacteristics and highly excellent battery characteristics evaluatedafter the testing of high-temperature storage durability.

When the aromatic compound other than those of Formula (2) was usedalone (Comparative Example 17-2), the initial rate characteristic wasdecreased from the level in Comparative Example 17-1. While the ratecharacteristic after storage was enhanced as compared to ComparativeExample 17-1, the improvement was smaller than that obtained in Example17-2. It is thus clear that the use of the inventive electrolyticsolutions provides excellent battery characteristics.

When the aromatic compound outside the category of the esters of Formula(2) was used alone or in combination with the aromatic compound otherthan those of Formula (2) (Comparative Example 17-3, 17-4), the initialrate characteristic was enhanced as compared to Comparative Example 17-1but the rate characteristic after storage was decreased. It is thusclear that the use of the inventive electrolytic solutions providesexcellent battery characteristics.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and aromatic compounds other than those ofFormula (2).

Example 18-1 and Comparative Example 18-1 Example 18-1 [Preparation ofNonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L. As an additive, 5.0 mass % ofmonofluoroethylene carbonate (MP2) was dissolved, thus forming a basicelectrolytic solution. Further, 3.0 mass % of the compound (2-2) wasadded. In this manner, a nonaqueous electrolytic solution of Example18-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 21 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 12 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolyethylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was allowed to stand at 45° C. for 72 hours.Thereafter, the battery was discharged at a constant current of 0.2 C to3.0 V. Next, the battery was CC-CV charged at 0.2 C to 4.35 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3.0 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas CC-CV charged at 0.2 C to 4.35 V (0.05 C cutoff) and was dischargedat 0.2 C to 3.0 V, thereby determining the initial 0.2 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.35 V (0.05 C cutoff). Thereafter, the battery was immersed in anethanol bath, and the volume of the battery before the testing ofhigh-temperature storage durability was measured based on the buoyancy(Archimedes' principle). Thereafter, the battery was stored at a hightemperature of 60° C. for 7 days. After being cooled sufficiently, thebattery was immersed in an ethanol bath, and its volume was measured.Based on the volume change before and after the high-temperature storagedurability test, the storage gas production was determined. Next, thebattery was discharged at 25° C. and at 0.2 C to 3.0 V, and the capacityremaining after the testing of high-temperature storage durability wasmeasured. The ratio of the capacity to the initial 0.2 C capacity wasdetermined as the residual ratio (%).

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing of high-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.35 V (0.05 C cutoff) and was dischargedagain at 0.2 C to 3.0 V. The ratio of the discharge capacity to thecharge capacity obtained during this process was determined as therecovered 0.2 C efficiency (%).

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 18 relative to the results of ComparativeExample 18-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 18-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 18-1, except that the electrolyticsolution of Example 18-1 did not contain the compound (2-2).

TABLE 18 Initial Storage Recovered 0.2 C gas 0.2 C capacity/ produc-Residual efficiency/ Additive % tion/% ratio/% % Ex. Compound 3.0 100.3356.7 109.74 109.33 18-1 (2-2) mass % Comp. — 100.00 100.0 100.00 100.00Ex. 18-1

From Table 18, the use of the nonaqueous electrolytic solution ofExample 18-1 according to the invention resulted in an excellent initial0.2 C capacity, a small storage gas production, a high residual ratioand an excellent recovered 0.2 C efficiency evaluated after the testingof high-temperature storage durability, as compared to when thefluorine-containing cyclic carbonate had been used alone (ComparativeExample 18-1). That is, the use of the inventive electrolytic solutionsmakes it possible to obtain batteries having excellent initial batterycharacteristics and excellent battery characteristics evaluated afterthe testing of high-temperature storage durability.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) and fluorine-containing cyclic carbonates.

Example 19-1 and Comparative Example 19-1 Example 19-1 [Preparation ofNonaqueous Electrolytic Solution]

In a dry argon atmosphere, LiPF₆ as an electrolyte was dissolved into amixed solvent including ethylene carbonate (EC), ethyl methyl carbonate(EMC) and diethyl carbonate (DEC) (volume ratio 3:4:3) so that itsconcentration would be 1.2 mol/L. As additives, 5.0 mass % ofmonofluoroethylene carbonate (MP2), 2.0 mass % of 1,3-propanesultone(MP8) and 3.0 mass % of adiponitrile (MP5) were dissolved, thus forminga basic electrolytic solution. Further, 2.0 mass % of the compound (2-2)was added. In this manner, a nonaqueous electrolytic solution of Example19-1 was prepared.

[Fabrication of Positive Electrode]

In N-methylpyrrolidone solvent, 97 mass % of lithium cobalt oxide(LiCoO₂) as a positive electrode active material, 1.5 mass % ofacetylene black as a conductive material and 1.5 mass % ofpolyvinylidene fluoride (PVdF) as a binder were mixed together with useof a disperser to give a slurry. The slurry was uniformly applied toboth sides of a 21 μm thick aluminum foil, and was dried and pressed. Apositive electrode was thus fabricated.

[Fabrication of Negative Electrode]

A slurry was prepared by mixing a natural graphite powder as a negativeelectrode active material, an aqueous dispersion of sodiumcarboxymethylcellulose as a thickener (sodium carboxymethylcelluloseconcentration: 1 mass %), and an aqueous dispersion of styrene butadienerubber as a binder (styrene butadiene rubber concentration: 50 mass %)with use of a disperser. The slurry was uniformly applied onto a surfaceof a 12 μm thick copper foil, and was dried and pressed. A negativeelectrode was thus fabricated. In the dried negative electrode, the massratio of natural graphite:sodium carboxymethylcellulose:styrenebutadiene rubber was 98:1:1.

[Fabrication of Secondary Battery]

The positive electrode and the negative electrodes described above, andpolyethylene separators were stacked in the order of the negativeelectrode, the separator, the positive electrode, the separator and thenegative electrode, thus forming a battery element. The battery elementwas inserted into a bag which was made of a laminate film of aluminum(thickness 40 μm) coated with resin layers on both sides, while ensuringthat the terminals of the positive electrode and the negative electrodesextended beyond the bag. Thereafter, the nonaqueous electrolyticsolution was poured into the bag, and the bag was vacuum sealed. In thismanner, a sheet-shaped nonaqueous electrolyte secondary battery wasfabricated.

[Evaluation of Initial Battery Characteristics]

While being pressed between glass plates, the nonaqueous electrolytesecondary battery was charged at 25° C. and at a constant currentcorresponding to 0.05 C for 6 hours and was discharged at a constantcurrent of 0.2 C to 3.0 V. Further, the battery was charged at aconstant current corresponding to 0.2 C to 4.1 V and then at theconstant voltage (such charging is also written as “CC-CV charging”)(0.05 C cutoff) and was allowed to stand at 45° C. for 72 hours.Thereafter, the battery was discharged at a constant current of 0.2 C to3 V. Next, the battery was CC-CV charged at 0.2 C to 4.35 V (0.05 Ccutoff) and was discharged again at 0.2 C to 3 V. In this manner,initial battery characteristics were stabilized. Thereafter, the batterywas CC-CV charged at 0.2 C to 4.35 V (0.05 C cutoff) and was dischargedat 0.2 C to 3 V, thereby determining the initial 0.2 C capacity.

Here, 1 C indicates a value of current at which a reference capacity ofa battery is discharged in 1 hour. For example, 0.2 C indicates acurrent that is ⅕ of the 1 C current.

[Testing of High-Temperature Storage Durability]

After the evaluation of initial battery characteristics, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at 0.2 Cto 4.35 V (0.05 C cutoff). Thereafter, the battery was stored at a hightemperature of 60° C. for 7 days. After being cooled sufficiently, thebattery was discharged at 25° C. and at 0.2 C to 3 V, and the capacityremaining after the testing of high-temperature storage durability wasmeasured. The ratio of the capacity to the initial 0.2 C capacity wasdetermined as the residual ratio (%).

[Evaluation of Battery Characteristics after Testing of High-TemperatureStorage Durability]

After the testing ofhigh-temperature storage durability, the nonaqueouselectrolyte secondary battery was CC-CV charged at 25° C. and at aconstant current of 0.2 C to 4.35 V (0.05 C cutoff) and was dischargedagain at 0.2 C to 3 V. The capacity of the battery after the testing ofhigh-temperature storage durability was measured and was expressed as aratio to the initial 0.2 C capacity. In this manner, the recovery ratio(%) was determined.

The nonaqueous electrolyte secondary battery fabricated hereinabove wastested to evaluate the initial battery characteristics, thehigh-temperature storage durability and the battery characteristicsafter the testing of high-temperature storage durability. The evaluationresults are shown in Table 19 relative to the results of ComparativeExample 19-1 taken as 100.0%. The same applies hereinafter.

Comparative Example 19-1

A nonaqueous electrolyte secondary battery was fabricated and tested inthe same manner as in Example 19-1, except that the compound (2-2) usedin the electrolytic solution of Example 19-1 was replaced by 2.4 mass %of the compound (3-1). The substance amount of the compound (3-1) usedin Comparative Example 19-1 was the same as that of the compound (2-2)used in Example 19-1.

TABLE 19 Initial 0.2 C Residual Recovery Additive capacity/% ratio/%ratio/% Ex. 19-1 Compound 2.0 100.6 101.9 101.37 (2-2) mass % Comp.Compound 2.4 100.0 100.0 100.00 Ex. 19-1 (3-1) mass %

From Table 19, the use of the nonaqueous electrolytic solution ofExample 19-1 according to the invention resulted in an excellent initial0.2 C capacity, a high residual ratio and an excellent recovery ratioafter the testing of high-temperature storage durability, as compared towhen the aromatic compound outside the category of the esters of Formula(2) had been used together with the fluorine-containing cycliccarbonate, the sulfur-containing organic compound and the cyanogroup-containing organic compound (Comparative Example 19-1). That is,the use of the inventive electrolytic solutions makes it possible toobtain batteries having excellent initial battery characteristics andexcellent battery characteristics evaluated after the testing ofhigh-temperature storage durability.

The above results have confirmed that battery characteristics arespecifically improved by the synergetic effect of the combined use ofthe esters of Formula (2) with fluorine-containing cyclic carbonates,sulfur-containing organic compounds or cyano group-containing organiccompounds.

INDUSTRIAL APPLICABILITY

The nonaqueous electrolytic solutions of the present invention allownonaqueous electrolyte secondary batteries to achieve high initialbattery characteristics and excellent battery characteristics afterdurability testing, making it possible to reduce the size of and toenhance the performance and safety of the nonaqueous electrolytesecondary batteries. The nonaqueous electrolytic solutions and thenonaqueous electrolyte secondary batteries of the present invention maybe used in various known applications, with specific examples includingnotebook computers, pen-input computers, mobile computers, electronicbook players, mobile phones, mobile fax machines, mobile copy machines,portable printers, headphone stereos, video movie machines, liquidcrystal televisions, handy cleaners, portable CD players, mini-discplayers, transceivers, electronic organizers, calculators, memory cards,portable tape recorders, radios, back-up power supplies, motors,automobiles, motorcycles, motor bikes, bicycles, lighting equipment,toys, game machines, watches, power tools, electronic flashes, cameras,load leveling power supplies, natural energy storage power supplies andlithium ion capacitors.

1-12. (canceled) 13: A nonaqueous electrolytic solution, comprising anelectrolyte, a nonaqueous solvent and an aromatic carboxylate esterrepresented by formula (1):

wherein A¹ is an optionally substituted aryl group, n¹ is an integer of1 or greater, R² and R³ are each independently a hydrogen atom, ahalogen atom or an optionally substituted hydrocarbon group having 1 to12 carbon atoms and may be bonded to each other to form a ring whereinwhen a plurality of R²s are present, R²s may be the same as or differentfrom one another and when a plurality of R³s are present, R³s may be thesame as or different from one another, a¹ is an integer of 1 or 2, whena¹ is 1, R¹ is an optionally substituted hydrocarbon group having 1 to12 carbon atoms, when a¹ is 2, R¹ is an optionally substitutedhydrocarbon group having 1 to 12 carbon atoms and A's may be the same asor different from each other, when n¹ is 1, at least one of R² and R³ isan optionally substituted hydrocarbon group having 1 to 12 carbon atoms,and when n¹ is 2 and R²s and R³s are all hydrogen atoms, R¹ is anoptionally substituted aliphatic hydrocarbon group having 1 to 12 carbonatoms. 14: The nonaqueous electrolytic solution according to claim 13,wherein a¹ in formula (1) is
 1. 15: The nonaqueous electrolytic solutionaccording to claim 13, wherein A¹ in formula (1) is a phenyl group. 16:The nonaqueous electrolytic solution according to claim 13, wherein thenonaqueous electrolytic solution contains the aromatic carboxylate esterrepresented by formula (1) in 0.001 mass % to 10 mass %. 17: Thenonaqueous electrolytic solution according to claim 13, wherein thenonaqueous electrolytic solution further comprises at least one compoundselected from the group consisting of fluorine-containing cycliccarbonates, sulfur-containing organic compounds, phosphonate esters,cyano group-containing organic compounds, isocyanate group-containingorganic compounds, silicon-containing compounds, aromatic compoundsother than those of formula (1), cyclic carbonates having acarbon-carbon unsaturated bond, carboxylate esters other than those offormula (1), cyclic compounds having a plurality of ether bonds,monofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts. 18: The nonaqueous electrolyticsolution according to claim 13, wherein the nonaqueous electrolyticsolution comprises 0.001 mass % to 20 mass % of at least one compoundselected from the group consisting of fluorine-containing cycliccarbonates, sulfur-containing organic compounds, phosphonate esters,cyano group-containing organic compounds, isocyanate group-containingorganic compounds, silicon-containing compounds, aromatic compoundsother than those of formula (1), cyclic carbonates having acarbon-carbon unsaturated bond, carboxylate esters other than those offormula (1), cyclic compounds having a plurality of ether bonds,monofluorophosphate salts, difluorophosphate salts, borate salts,oxalate salts and fluorosulfonate salts. 19: A nonaqueous electrolytesecondary battery, comprising: a negative electrode and a positiveelectrode capable of storing and releasing lithium ions; and anonaqueous electrolytic solution comprising an electrolyte and anonaqueous solvent, the nonaqueous electrolytic solution being thenonaqueous electrolytic solution according to claim 13.